If at all possible, I would recommend saving up to purchase a 6600GT. Zipzoomfly has the eVGA 6600GT here
for $184.99, shipping included. That would probably be the best option.
If you have to have something right now for under $100, the 9600XT on ebay would probably be the best thing you could find. I can't imagine a better card for under $100.
As they say, you get what you pay for, and I have a feeling you will not enjoy a sub-$100 card as much as you would a 6600GT, which I will soon purchase for myself
In conclusion, my advice is to wait until you have more money, otherwise I bet you'll feel disappointed.