He mostly plays Sims 2, The Movies, and other sim-type games. One of the big killers seems to be things like moving water, which at highest settings is designed to shine and sparkle and reflect semi-realistically. The level of realistic detail has gone up orders of magnitude between prior and current versions, which is why I'm speculating it's the video card slowing him down -- sound about right?

The Movies also seems to be crashing his machine, which is why he didn't play much after the first week or two. I'm not sure if that's a video issue, or a general RAM issue -- I believe he's got 512MB right now.