The 8600GTS is similar in DX9 performance as my X1950Pro, i also have Core 2 Duo 1.8Ghz and 2Gigs of RAM.
So i can tell you
.
First off just buy the 2nd Counter Strike Source package, it comes with DoD and CS:S.
Right Counter Strike and DoD run at maximum settings, filtering and AA at high, at about 60 frames per second for me. (very smooth) at 1280x768.
I also run HL2 at the same settings, except its about 50 Frames per second.
FEAR, hmm, i run that at 1024x768 at High settings (But Not maximum, AA, and Filtering turned down with one or two otehrs just below Maximum) and i get 40 Fps, and sometimes drops to 30 Fps, so its playable, but you can sometimes notic ethe lag.
As for Oblivion, not sure. ive just ordered that game. Theresalot of settings to play with though. You will probably have it between medium and high, although you could probably have it in high if you reduced the distance that object appear. Also turn off Grass shadows and HDR if you want better performance. I think if you go on TweakGuides, they give you an indepth way to set up oblivion.
have fun
. There all great games.
day of defeat source 
Linear Mode
