because 2 gb of ram isn't enough. i'm sitting right now on a c2d 1.8 with 2gb ram, and it has only a 7500le for that game, but it runs good enough on high-med settings. maybe vista takes too much cpu power from your (singlecore?) processor
7600GS should run CS:S and 1.6 just fine - I've run it okay on the ATI integrated mobos, bobwya you are just being a snob My third rig still plays Everquest @ 1280x1024 on a 7600GS just fine.
Presuming that you are running a sane resolution, I would suggest more RAM (hell as it's so cheap now I'd suggest more RAM to anything if you have the cash spare).
After that you need to isolate the cause of the problem - how busy does the task manager say your CPU is whilst playing? What version of the nVidia drivers do you have installed? Are other games about what you would expect? Is there a broken fan / loose heatsink on your GPU?