Camera body or lens = chicken or the egg I am wanting to upgrade to an FX camera (ideally the new d800) and will need to upgrade lenses to go with it. I currently have D90 with good primes and standard inexpensive zoom. The new camera and body are a significant investment that I don't think I will be able to do at one time. So the question is, do I purchase the new camera body first to use with a couple of primes or purchase a new quality lens (24-70 2.8) first to use on the old body?
Lenses suitable for FX will be a long road of saving and acquiring and will eventually greatly overshadow the cost of any body.. Most people upgrading to a D800 do not see much difference between well shot images with any modern DSLR. When someone wants a copy of one of your photos, it will not be because of any technical difference between your D90 and a $3000 D800. More great shots are taken with D90s than will ever be taken with D800s simply because so many more people are shooting with a D90 and everyone gets lucky or has a good eye regardless of their hardware. I have a D90, D7000 and D800 and like them all. I shoot most with the D800 but have more shots with the D90 hanging on gallery walls than either of the other two. What makes an image compelling has very little to do with resolution or DR, but instead on the scene framing, light, and interest of the story being told. So, with all this, I caution you that the very cheapest part of shooting FX is the body and to get lenses up to the quality of the D800 that cover the broad range of the kit zoom(probably 18-105vr) will require an investment of $5000 or more plus the camera body. If you want reach the price jumps very quickly compared to the same field of view on DX. My suggestion is if you really want to move to FX regardless of whether it generates images of higher interest, then be prepared to invest over a period of time $10,000 for general purpose photography and twice that for small birds in flight. Set a long term goal and start working on getting one lens at a time, all of which will work well on your D90 and when you are ready in a few years, you will have the lenses for the D850 or whatever which will be a more manageable addition. So what is your budget? What are the uses of the images? Are you going to be making a lot of $50-100 prints(if not.....why get a D800 in the first place, a D600 or D7100 DX would do just as well for prints smaller than your car). What specific lenses you get, since all good lenses are specialized with narrower focal range than general purpose superzoom lenses, will depend on your subjects and lighting conditions. One thing to ponder, why are you not happy with the D90 and current lenses? Do you have specific weaknesses that are overcome by investing in world class FX? I have seen almost no image defects that were corrected solely by changing cameras. If low light is the reason, either learn more about light management, or a better tripod, or faster primes. Even with the great D3s, the low light king in 12mpx, the dynamic range of a high ISO shot quickly loses out to a well exposed D90 shot. Yes, it can capture an image in low light but it is not good quality and useful other than by the fact that it is a miracle that anything was captured...it is not clean enough, or with enough color fidelity or detailed enough for any commercial purposes or hanging on your wall. If you want detail, color fidelity and clean shadows you are going to be shooting at ISO 100-200 with any of these cameras. If you have decided that your future is FX. Start by buying lenses that shine on your D90. A 70-200 2.8VrII is the best bang for the buck at $2400 and is owned by every serious amateur or pro. Next, the 24-70 2.8 is a must have. A fast quality prime or two, like the 85 1.4G and 24 1.4G. These will lighten your wallet by $7800. Next add a wide angle and if shooting wildlife or birds, a 300 2.8 plus a TC2III or the relatively low cost recently announced 80-400 update with VRII a TC. Or do what pros would do, fix the lighting situation. Although I love my D800, probably the most requested image I have taken that hangs on more livingroom walls was taken with a D90 and 18-105VR. Great lighting trumps great lenses and cameras every single time. Luckily lighting cheap and accessible to everyone who is a little creative in their approach to a subject.
Personally I'd choose the lens. The d90 is still an awesome body and the 24-70 lens takes amazing pics. You state you have good primes and a 'standard inexpensive zoom' well think how amazing your pics would be with this lens! Get the lens and at a future date sell your subpar zoom and eventually your d90 and use that $ to pay for an FX body later. Need further convincing? Stop by your local photo store (wolf, ritz etc), take your d90 body with you and take a few test shots with the 24 70 lens. You'll see the potential of how much better the photos you're currently capturing with a standard (kit lens? Off brand? What do you mean by standard?) lens could have been. Good luck!
It depends on your need. Usually I recommend get the good lens first BUT your need may be for something else. For me the need was low-light sensitivity for night-time sports so I bought the Nikon D700 *before* I had any high quality full-frame zooms. I had a few prime lenses that worked well and consumer grade full-frame zoom (Nikkor 70-300 mm which is really pretty good but at 5.6 not very good for night sports).
Once I married the Nikkor 24-70 mm f2.8 to the Nikon D700 camera, holy smokes, that was amazing.
To be honest, these days I 'test drive' any lens and/or camera body by renting it for a few days from LensRentals http://www.lensrentals.com With equipment running into multiple thousands of dollars sometimes, laying down a couple of hundred bucks to use what you intend to buy for a few days to see how well it works for you is a pretty good way to go. And FYI, I am no way affiliated with LensRental, but just putting the idea out there