Not sure exactly what "some gaming" means, but it sounds like it won't be anything too hardcore.
If you want relatively new technology that doesn't have the greatest 3D gaming performance, but has a nice price, you could pick up a 64MB ATI Radeon 9000, for about 65 bucks last I saw.
If he's looking to spend about exactly $100, there are Radeon 8500LE 128MB cards that will perform better, and will probably stay viable longer, because of the larger amount of ram, and you can find these for like $99 plus shipping at many online dealers.
As far as NVIDIA choices, I'll let someone else handle that, as I don't know much about them.
As far as dealers, check out Newegg.com and Googlegear.com.
they're two of the most consistent and trusted dealers.
hope that helps,