I can not go into details here. We should be seeing official performance results with Doom3 in the next couple of days and apparently ATI doesn't do too well. I think we already knew this. And if Nvidia's cards are still using a different rendering path instead of the default ARB2 path which is the default DX9 path, then they will of course out perform ATI.
Also keep in mind that Nvidia is a better OGL performer than ATI (at this time). I think we will see just the opposite results with HL2.
Also, the min system requirements are quite high. Like a 1.5, Radeon 8500/GF4, 512MB, 2 Gig of space, etc.