CS:S is pretty cool. But no, VAC rarely EVER works. Its not like an insta-ban for those aimbot speed hacking 12 year-olds that want attention because their parents dont love them... Ahem, I digress. Anyway, it could be the fact that your GFX card only has 128MB onboard memory. I am running an Nvidia 6600GT (128MB onboard) 1.5 GB Corsair XMS RAM, P4 @ 3.0GHz (no oc), and I am sort of struggling to play oblivion at 10248762 (widescreen, the res is close to that, not exactly sure), on meidum settings. I am not going to upgrade my video card until the DX 10 cards get A LOT cheaper, because I don't have tons of money to throw away. But them, my CPU will be a big bottleneck for my system, so I will need a new MoBo and CPU. Man computers can cost a lot... >.>
EDIT: I can play CS:S at 1440x900 most settings High (No AA) at around 40 fps, which isn't too bad. So 128bit cards don't make it look THAT bad 😛