Do you have a preference between ATI and Nvidia? I'm guessing not, but you never can be sure.
ETA:
Anandtech's Holiday 2011 Budget PC guide is probably a good place to start. They recommend AMD/ATI cards as the best bang for your $100 budget.
Thanks for the response and for the link. As you pointed out, I have no preference. Actually one of the reasons I came here to ask about it was in case there was wisdom about which is better. The guy doing the upgrade asked me which I liked better, and I, of course, have no idea. Making a note of the AMD/ATI recommendation, thanks again.