Yeah, I had this problem when playing Red Alert. It's definintly the video card. I'd get myself a better one, but I don't want to waste my money when I'm going to eventually get another computer. (I use the word "eventually" lightly)
Dude that should work fine. I had my friends PCI 7500 64MB work fine with 64-bit Memory. When your trying to play CS:S, Put your desktop to 16-bit so the game has more GPU power. Take some of the styles off your computer to get some more ram and GPU power