prey is also far better than doom 3 and quake no annoying spawning of enemies in stupid places. Condemned is alright more of a horror/fps like F.E.A.R Focuses on melee combat more and if you get a gun you only have so many bullets etc.
6600 is normal video for games like Prey (Doom3 engine) or Condemned (FEAR engine), I think you could play these games with at least medium setting and 800x600 it's no so bad
I downloaded prey demo, and the game runs for me better than Quake4 and doom3. I play it at 1024x768 on Max setting (just AA and AF are off). The graphics are very realistic, and gameplay is excellent.
When i choose Auto detection it sets it at 1024x768 on max setting and AA and AF (image antistrophy) are off, and thats is just my default setting.
When I run it with 2xAA and 4xAF, the game still run fine on my system.
I don't know but maybe it runs better for me because i have better cpu and more ram than your computer. Also, my video card is overclocked from 300/500 to 345/570.