Interior design, architectural, real estate etc. gear?

First question is what do you feel you're not getting with what you already have? The D7000 is a most capable camera and does very well in low light, so what are you trying to accomplish? Beauty shots? Web and pennysaver ads? What are you being asked to provide for your clients?

Laura, IMHO you need to start with a tripod, a good one, and later you can go for a wider lens if necessary.

I would also recommend looking into a tri-pod, and when you decide to look into lenses, the Nikon 10-24, Sigma 10-20, Sigma 8-16, and Nikon 12-24 are all worth taking a look at.

Tokina 11-16 is real nice, too. If you need versatility in this lens, though, get one that goes out to 24mm. The Tokina 11-16 and the Sigma 8-16 are both SO wide they are not "stay on" lenses for any length of time, imho... unless you have a very interesting view of the world...

+1 on the suggestion about a tripod -- it's an essential. Ive shot a lot of these kinds of shots, and to get started I'd practice shooting on the tripod at high ISOs and long shutter speeds. I used the 12-24 and the Nikon 10, and find the 12-24 to be more productive if I only had one. Learn how to bracket exposures as well, since you'll find yourself in many situations where you need to balance interior and exterior light, with or without a flash. Layering multiple exposures or having Photoshop merge them in its own HDR Pro is pretty easy, but you have to start with the correct exposures.

The good thing is, you can practice at home ;-)

As architectural shooting requires straight lines and minute control over perspective, I second all the other suggestions of getting a tripod. The legs don't matter too much for what you are doing, but you are going to want to get the best head that you can afford. Cheaper ball/pan heads are tough to lock down accurately, and nothing is more annoying than getting your camera perfectly level, locking down the head, and then having it droop a degree or two.

As for lenses, you will probably want something wider eventually, but your kit lens will deliver great results at the apertures you are working at. If you want to go wider without buying a lens, there are tons of great photostitching applications out there that will let you combine multiple shots into a single image. Not only will this allow you to capture wider angles than you can currently, but it will also allow you to capture more detail.

Good luck and have fun with your photography.

Using flash to light a room is difficult. A pro might use 5 or more strategically placed remote flashes. Best to stay with existing light for now. A tripod will usually be necessary.

If you have windows in the shot, you will need to balance the exterior and interior. You can do it in the computer with HDR. Or, twice a day (at dawn and dusk), the interior and exterior will be balanced in brightness for about 10-15 minutes. I use the later technique when I shoot my own house. When shooting someone else's house, it may not be convenient.

Thanks, everyone!
My first location is a vacation home which needs pictures for ( perhaps web? mostly paper) ads. I'd say beauty shots for the first one. This is a high level expensive vacation home. The other location is a hospice care facility; again, shots for web and flyer ads.
For lenses I'd like to have something with a slightly wider angle that I could also use for landscapes. Then again I still need to think about it.
I do have a tripod, it is old however. I'm guessing from the 80's or 90's. It's pretty basic, sturdy but very.. stiff at the same time.
So... What brand? What model? And flash or no? I've also been looking at getting a battery grip for a year. But the Nikon is so expensive...
Edit: I was thinking about layering pictures in Photoshop. I would use the flash in different places and put them all together... What do you think? Is it possible without for example Pocketwizards?

Small flash heads provide light over a (small) specific area... using the flash in different points will give you different small illuminated areas; you can use it reflected in the ceiling or walls to have broad illuminated areas... and then to work on layers... too much complicated, specially with a wide angle lens. Simply use your tripod and available light. The flash head will serve you to fill a shadow, or to emphasize an specific area. An small soft box will provide shadowless illumination on an area proportional to the size of the softbox... good for a head portrait, aproximately. Not for big areas.
Your tripod, if sturdy and functional, is fine. It`s like a table, if it works, that`s all. Modern tripods are lighter, stronger, has better locks. If possible, try to clean and lubricate yours.
You need a bubble level to correctly set the camera. And if you buy a flash head, another tripod, grip or whatever to hold it outside the camera. You have to check what you need to release the flash from remote. A cable release is all what you need for the camera. I use a light blocking/IR filter on my camera`s built in flash to release remote flashes.
For this kind of work many times I have used a ladder; it depends on the specific task. The way to attach the camera to the ladder must be also anticipated.
I`d use that money just to buy a wider zoom and a few accessories. It seems to me that a third party wide angle zoom could be the solution.