What is the power supply exactly? a cheap one is a bad idea.
For the gpu itself, a HD6770 or 5770 is good, you might be able to get a 6790 i think. If you can, there is a 6850 for about that much that rebates down.
That is plain out bad advice. Sub $100 you are looking at a GTS 450 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814132005) or GTS 250 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814187099) both of which will far outperform the 9800 gt (the 250 is a reboxed 9800 GTX+)If you are looking to spend under US$100, then an NVidia 9800 GT is your best bet. For AMD (ATI/Radeon), we don’t have enough of these cards at this price point to get you good data. This would be the minimum card for enabling all the features, only at reduced quality settings.