ATI are at the moment producing better competitively priced products, and I believe run slightly more energy efficient, which does matter when it comes to a PC with a high wattage power supply.
I may be wrong, but I think ATI have the upper hand at the minute on the fastest card too?
I may have to go for a Nvidia card next though for my 3D based work, unless something starts using ATI's tech.
Nvidia has the upper hand in some technology that could be used a lot more later; thats for sure.
CUDA could be used for a lot of things once its potential is unlocked fully.
I had to learn the hard way that ATI doesn't look that great when it comes to video watching, especially in HD, the quality is much better on my old Nvida 8500gt. But yes for gaming I think ATI is bomb and for $155 my HD 5770 gave me 54 fps in GTA IV, I'm very happy.
Nvidia pays to get their name put in games, and in some rare instances games are actually optimized to run better on nvidia cards, but not too often. Physx is a nice little graphical demo thing, but will drop your framerate significantly and isn't always feasible for running games well
Kage offers good advice. The ATI HD5xxx series has the best card out right now (the HD5970 Hemlock) when it comes to gaming and multimedia. However, if you plan on doing any rendering/modeling, the ATI Fire lineup doesn't come close to nVidia's Quadro FX.