By the looks of it, your CPU and RAM is slowing you down more than the 6600! I ran HL2 and CS:S at 1024x768 (no AA or AF) at something like 25FPS+. HL2 is VERY CPU dependant. The graphics card won't nescessarily slow down when its underpowered, mine would just show the colors wrong, or restart the PC, or just hang.