Heyyo,
Now, we don't know his budget. Of course, if you can scretch it, get a 6600gt, it's a pretty good vidcard, just avoid I think the xfx versions of the agp cards that are the original build, the revised version is fine.
If I were you, just get a 9800 Pro 128MB. the diff between 256MB and 128MB is minimal. No games out right now are 256MB optimized. Oh, and if you can, check n' see if ti's an r350 core or r360. Obviously the r360 core will do much better than the r350.