It's an interesting question, as I also switch from times between ATI and nVidia.
My last ATI was X800XTPE, and it was a very good card, and it still is.
Today I have nVidia 8800 GTS 640MB, and it has been for 3 years a very good card also.
Of course my nVidia is better than my ATI since it is one generation newer (DX9 vs DX10 card), but if I want to buy a better card, I can't really find one.
nVidia's GTX 280 is not really worth the upgrade, since it doesn't bring any benefits over my existing card:
I can still run Crysis with satisfying speed at Very High mode.
The only reason for me to upgrade would be to look for future development, which means: 64-bit GPU.
The limits of 32-bit GPUs are really completely outdated. People are talking about GPU tesselation, and with a 32-bit GPU you can't even cast shadows further than a few kilometers away before they get unusable inaccurate at the projection position (you see the shadow being off by many centimeters when you're away from the light source about 1-2km).
Since my 8800 GTS supports OpenGL 3.2, but not fully OpenGL 4.0, the deciding factor is to get a 64-bit card which supports OpenGL 4.0 fully.
nVidia seems to have only 32-bit cards, while ATI has been pushing even to 128-bit GPU and 16/32 core CPUs for a while.
When there is a OpenGL 4.0 game engine on the market, I will buy it. And then I will need to buy a 64-bit GPU also,
and at the moment it seems only ATI can deliver that.