My X800Pro runs all games nicely on my 2405 except for FEAR. That should maybe give you an idea.
Doom 3, Far Cry, and HL2 are all very playable at 1920x1200, albeit with little or no AA. HL2 works fine with 2XAA, but I prefer Far Cry without it enabled due to the frame rate hit. That's with all settings to maxium, I might add (although Doom 3 is on "high", not "ultra" for obvious reasons).
My guess is that the 7800GT would run most games at 1920x1200 with 2XAA so long as you're not overly fussy (read: 60fps constantly). FEAR is a total writeoff and I suggest against buying hardware just for that game. Its graphics do not justify the hardware it craves.
Oh, one more thing to add: COD2 doesn't run well.