Upgrade to D700 or no?

What is wrong with your D90? Is there something the D700 will do for your photography that your D90 cannot? I can't answer your question without more insight.

Based on the fact that the OP even needs to ask this question in the first place, the answer is very obvious.

Apurva,

If you take lots of hand held photographs in dim light with your D90 then the D700 will give you even more low noise / high ISO performance than your current D90.

I have checked out your photo portfolio here on p.net, apart from some indoor travel shots and some indoor Cathedral shots and some worthy still life work I see you travel a bit and shoot mainly outdoors. You have taken some very nice photos with your current combination, I can't see where it might be holding you back.

The D700 will offer a slightly better auto focus unit which may assist with wildlife photography - maybe a D300 (which shares the same AF unit as the D700) might suit you better if you are happy to retain the 18-200mm super zoom which you appear to favour for travels.

If you wait 6 months the D700 might be "old hat' and superceeded by another camera in the FX entry level class........

Nobody ever asks how big you print.

If you don't print above 8 x 10, you will not see a difference between a well-shot image on FX vs. DX. Most people who want FX don't need it.

If you are generally happy with the images you are getting, you really don't need FX.

In short... if you don't KNOW you need FX you almost certainly don't need it.

If you use a lot of telephoto, you are better with DX.

If you have money to blow and just want to have fun, well... if that were me, I'd have a D700 if only for the bigger viewfinder. But I don't have money to burn.

Thanks Matthew for some informative and to-the-point answer. The rest of you, thanks for taking time to read my question but your answer is hardly any answer.
All that the OP is asking is 17-55 on D90 is as good as 24-70 on D700? I would be most obliged if someone gave me some fact or opinion based answer rather than going into my motives and wondering about my cedentials.
Of course there in nothing wrong with D90, but then Nikon should stop making new bodies. And none of the photographers in this world should ever get a new camera if he/she has D90 or anything like it!
And suppose I have money to blow, any problem out there?

Apurva - both the 17-55 and 24-70 have been reviewed at http://www.photozone.de/

I find this site's graphs to be very useful in evaluating lens choices for Canon. I'm sure you can get a solid idea of performance on these Nikon lenses.

Naturally you should check out the 24-70 on full frame review since that's how you plan to use it. Remember that you cannot compare the absolute resolution figures between different senors, but the graphs are labeled good/very good/excellent, and you can compare those labels to get an idea of how these lenses compare.

Part of the reason you got the answers you got is because you did not specify your shooting style or needs. It's hard to answer your question without knowing these things.

Good luck!

The reason you didn't get answers you like is because your original question was "Should I go for the D700" without telling us what types of photos you shoot. If all you are going to do is use the equivalent midrange zoom in good light and print 8x10 then you won't see much difference between the 17-55 on DX or the 24-70 on FX because both lenses are good and both the DX and FX sensors are good in good light. I think the lenses choices you want to use should drive the sensor size you pick. If you plan on a lot of telephoto stick with DX. If you plan on very low light, extreme wide angles or PC/tilt lenses then go with FX.

Personally I have a D3 that I use in low light with 28, 50, 85 f1.4 primes. The D3 also gets used for sports where I need the fast AF. I also have a D90 that I use with the 12-24, 18-70, and 70-300 consumer zooms for travel.

Apurva, I upgraded from the D200 to the D300s this year after several years of D200 ownership. The momentum from the buzz surrounding the D300s (and a new lens purchase) has me shooting a lot more now than I did last year. If it were me, I'd buy the D700.

I don't have any of the lenses in question nor have I used the 18-200mm. I suggest you check Bjorn's site as well for your research. http://www.naturfotograf.com/index2.html
I would say the D700 with 24-70mm is the best combonation but you pay considerably for that last percentage of improvement which may be hard to see.
I do think you have left out details of your needs for asking a question like this. If your NAS has an itch then by all means scratch it. I did even though a D300 would have met most of my needs. I do enjoy using it and will use it until it wears out, a personal requirement I set before purchase. One of the reasons I choose the D700 was to use manual wide primes as well as zooms. If you need the extra stop then go for the D700. If you travel alot then I would look closely at the Tamron 18-50mm f2.8 and get a great light tripod to carry also. Currently my D700 with the Tamron 28-75mm weights 4 pounds then add another two pounds for extras. That is 20 percent of my total overnight carrying weight including what I wear, glasses, shoes, socks, wallet, etc when hiking. I would be better off with a D90, 16-85mm weight wise.