Please help-which Nikon SLR is best for me:)

Sara, first of all:

• What is your budget?
• What type of subjects would you like to photograph?
• Do you want to shoot some video with the camera?

The D40 and D80 are both discontinued. They are only available used.

If you find the D90 on the pricy side, look into the D5000. Below that, the D3000 is currently the most economy model. I did review some of them, though:

• D5000: http://photo.net/equipment/nikon/D5000/review/
• D3000: http://photo.net/equipment/nikon/D3000/review/ There is a chart at the end of that review, comparing several of the Nikon consumer DSLRs.

Both the D90 and D5000 have video capture. If you buy a used D5000, make sure that it is not on the recall list or it has been properly fixed.

Finally, the camera body is not the end of the story. You will need a couple of lenses. Make sure you budget for them.

This is my simple take on the Nikon lineup...
The D40 series, this includes the D40(x), D60, D3000, and D5000. These were desisgned to be entry level bodies that were easy to operate and take great photos. As time and technology progressed they get better and better as does everything. Most advanced features require extensive use of the menues. They also don't have internal focus motors, they require an AF-S lens to autofocus. These bodies tend to be less expensive and smaller. Why they renamed the series from D60 to D3000 I don;t know. But the Dx000 is not the same as the D90 series.
The D50 series, this includes the D70(s), 80, &90. This series has the focus motor and will use older non-AF-S lens. They also have more dedicated buttons and knobs for advanced functions as well as a dedicated LCD screen for things like shutter speed and aperture. They cost more and give you more features. I started with a D70s and a D80. I had a bunch of older lens that needed the screw drive focus and likes the extra buttons and knobs. Thes bodies cost more and are a bit larger.
The D200 series, this includes the D300(s) and D700. Theis series has a more robust build and still more buttons and knobs. They don't have the full auto modes or scene selections the first two had and are consisteded semi pro or advanced amature. They are also heavy. I left my D80 and bought a D300 and now a D700 because I like the build quality and feature set. The D700 is a "full frame" camera and I like wide lenses, and I have some older ones that work well on it. It's included in this group because it fits except for the FX frame size. These are still affordable, although it hirts a bit, by the non-pro. They are heavy and the larger still than the others.
Pro Bodies, D1...D3(x), I don't have the pockets for these, the D700 gives me what I need out of this group. They are very large and robust and have the mother load of buttons and knobs.
So I guess to boil it all down you have to ask yourself the "do I need a focus motor or not" question. Video is included in the newest models in all but the Pro line and it will be there soon. And if the answer is yes then that only leaves the question of metal body or not. Two simple questions. Problem solved.
Notice I didn't mention anything about image quality. They are all dependent more on your skills in that area. Mega pixels are not the bigest deal in modern cameras. I have great photos with my D70s that is antique tecknology as compared to todays offerings. So I don't think that is as important as the other two questions. The image quality of a D5000, D90 or D300s are not that far apart for most of the pictures you will be taking.
Nikon has spent countless hours organizing the feature set and price point for each body. Some features may be very important to you for one model or another and be a deal breaker. Bit I still think it comes down to "focus motor or not" and "plastic or metal body". Very simple.
I still have the D80, it's light and takes great pictures, easy on the neck.
I have a D300, I use it mostly for long lenses, so I guess its a digital teleconverter built into a body. I also have an 18-200 that I use most of the time on it. My grab and go combo.
I have the D700, great at just about everything. Mostly wide angle and indoors.
All mine have the focus motor because I have older lenses that need them. I also like the dedicated buttons and knobs. All use the same batteries, a real bonus, thanks Nikon for that.

They just make arbitrary numbers up as new cameras are needed in the line up imo.

Sara,
I won't profess to know all this as well as others on the site but here goes:
Lens Selection
If you rely on auto focus or camera metering (most likely) Nikon has created a few distinct levels with its cameras. Cameras such as a D40, D60, D5000 auto focus when matched to AF-S lens. Focus on the AF-S lens occurs in the lens itself.
A D90 and other can use the AF-S lens plus many older lens because the camera body includes a focus motor. This makes the camera body a little more expensive but then Nikon adds additional features to make this level of camera somewhat better.
Aftermarket lens selection, or used lens, provide a greater range of options with the cameras designed to take advantage of either type of lens. So its possible to buy a D90 with a used lens or third party lens for a reasonable price.
Flash feature
All the cameras will work in iTTL mode but the lower level cameras will only control an additional flash if attached to the hot shoe or by a cable. The mid-level and above cameras can control a flash remotely. So, if you buy an SB600 Nikon flash you can set the D90 to operate the flash remotely. The exact same flash on a D40 works - quite well -but only in the hot shoe or with a cable. A cable can cost $50 to $80 additional if you want off-camera connection.
Other Considerations
Focus system (number of focus points), size of viewfinder, low-light performance ( ISO 1600 can look just like a cheap point & shoot on low tier models) , physical size, etc. You may also enjoy a newer models featuring the video ability. Don't forget to check the Nikon web site for specific specifications on current models.
I keep seeing used D40s offered for about the cost as when new so don't count out a new Nikon camera kit. D90 may fall into the same category regarding price.
Mavins Jump in here and save me
OK. Some else fill out the rest regarding D80, D200, etc, etc, to help Sara. Plus, a good welcome to the site.

Sara, welcome! Another point is some of the older models like the D200, (D40 also?) which is what I use, have the CCD sensor and while this is great for me, it doesn't handle the low light/higher ISO settings above say 800 all that well. Again this is personal. I shoot everything at 125 and 320 sometimes 600 like I'm used to with film, so for me it's perfect. D90 is a real nice machine, I'm waiting for it to get disc. so I can get one cheaper ;). BTW, I've been shooting Nikon for almost 30 years and only once have I bought a brand new just introduced model. I usually wait for the disco and then price hunt.

i'll make this simple: just get a D5000. it has the same image quality and low-light performance as the more expensive d90 and d300.

Get a camera that will have some uncharted functions and territory, or else you may reach the limit of its capabilities sooner than you expect. The more controls the merrier, as they will allow you to learn about exposure slowly and through practice. The basic cameras with do-it-all features will make you feel secure and comfortable, but you won't learn much from them.

Also, as Shun pointed out, before getting any valuable pointers you should indicate what your budget is, and the type of photographs you take. Do you have a flickr page? Just post a link... And take more photographs!

A used D90 is just a little more than the lower end cameras are new. D90s are selling in the $650 - $700 range on eBay making them highly affordable and an exceptional value - you get a lot of camera for the buck.

The D90 has just about everything a photographer could want/need.

Since Sara did mention that the D90 is on the expensive side, she is clearly getting a starter DSLR. I think it is best that we provide simplier answers for people who are new to Nikon DSLRs.

If you want more manual controls, the D90 and D80 are better for you. In the last few years, Nikon has introduced a series of very small DSLRs that use the EN-EL9/EN-EL9e batteries: D40, D40X, D60, and D3000, The D5000 is a bit larger but also falls in this category. There is no room on those cameras for a lot of buttons. Therefore, a lot of the manual controls are burried under menus. Most of the controls are still there; they are just harder to reach.