Some of us have lusted after the D3 and D700 since they were announced. They set a new high bar for cleanliness at high sensitivity, in fact you can use their files at 6400 if you are reasonably skilled with post processing.
This seems like some sort of Nirvana, where the equipment solves half your problems. But, alas, that's not really true. Skill always trumps equipment .. every time. So the rich doctors and dentists may have the best toys, but they don't get the best images. (And before you become outraged at my arrogance, I am not implying that I get the best images either, so swallow that bile.)
That said, there is no doubt that the equipment makes it easier all the time. The APS-C dSLRs have gotten better from generation to generation and it is now commonplace for people to shoot at 3200 ISO if necessary, whereas in the days of the D70, one had to have serious cojones to shoot at 1600 ISO. I didn't mind doing that, but then I am a RAW shooter and I work on getting excellent exposures.
So ... how good is the D300 at 3200 ISO then? Is it usable? Well, I think it is, actually. Especially when you are not actually shooting in the dark. The high ISO for shooting in the dark thing is a misnomer anyway ... we don't buy these cams so we can shoot literally in the dark (although they are certainly used that way when necessary), we buy them so that you can maintain decent shutter speeds as the light dims.
Sweet light tends to be dim light ... near sunrise and sunset, and shutter speeds fall faster than you might imagine ... your eyes adjust transparently, being logarithmic and all. But that poor sensor responds linearly, which means that cutting light in half is a serious thing.
So to shoot people or wildlife at those times, you need higher ISO.
Case in point -- in early June, I had the pleasure of attending a conference in Florida and we got to go to the Animal Planet park late one evening. By the time I got half way around the park, the light was noticeably dim ... and there were a lot of trees, so it got very dim at times. I was shooting the wonderful 70-300VR, which has a wide open aperture of f5.6 at full zoom, which is decidedly slow (consumer zooms tend to be that way.)
So the only choice was to crank ISO up to 3200 and hope. And I was not disappointed at all. In fact, some of the images surprised me greatly, as they came out very smooth and detailed. I was so pleased ...
Two examples: a snake attached to a lady in a dark area, and one woman painting the face of another in an area that was one stop brighter (according to the two exposures.)
And here are 100% crops taken from these two images with no noise reduction performed over and above what was done during RAW conversion. No extra processing in photoshop at all in fact.
Click on these to see the larger versions and I think you'll agree that the camera can do the job.
So ... does everyone need a full frame camera? Well, no. That's actually obvious. The current generation handles high ISO just fine, especially in moderate light. In fact, the D90 is slightly better than the D300 and the D5000 appears to be slightly better than the D90. Banner days for Nikon shooters, that's for sure. (For some reason, Canon is pursuing higher pixel counts and it has hurt them in this area ... I really hope that Nikon avoids taking a hit on high ISO quality so soon after they achieved parity with Canon -- and some say they have the lead, in FF I think this is true.)
And remember that all your lenses drop reach by 33% on full frame, and your DX lenses drop the cam down to 7mp ... not so rosy if you don't already own pro glass. So concentrate instead on nailing your exposures and on adding light. You won't regret the time spent learning the ropes ...