Depends what you're into. Lately, I've really gotten back into Company of Heroes, and you could probably squeak that, depending on your resolution. I've played a good bit of Battlefield 1942 with a Sempron 2800+ with the onboard video, and that's always a fun game too.
Should be able to play World of Warcraft on low-ish settings...
pretty low... or grab a used 4650 agp and play wow at med+ settings.
I hear diablo 2 is good. Neverwinter nights was one of the best games I have ever played. You can download them via torrents and get all of the expansion packs as well.
Most old school games can be had via isohunt