9800GT requires at least 400W. GT 630 should run, but it requires 300W so it will be really close... Is upgrading the PSU an option?
At this time, no. He's pretty dead-set on the $100 budget, if I can't do it for that he's just going to wait till next year and do a full build. I'd considered a 630 but the 9800 actually performs better (or so I hear) in DX9 games. Looks like that'll have to be the way to go, though.
I would probably look at a "silent" or passively cooled card. Not only do they tend to use less power, but since you're dealing with an OEM case, anything you can do to keep the heat output down the better.
Are there any actually worth buying? Most of the ones I've seen use horrible memory, and I'm not sold they're worth the upgrade cost.