I would be looking at a GTX 460 of some sort or a radeon 5750 or 5770. They're all quite a bit faster but honestly, you're dipping into that territory in which saving a few bucks drastically reduces your performance. Deals pop up for these cards from time to time if you don't mind doing rebates. If you want a LOT more performance, there are 5850's floating around for ~~ 140$. GTS 250 would be a fair bit faster too but you rarely see those pop up anymore. same for 4850, 4870. So yes, a 460 or a 5750/5770 can be found for under 100. Try for a 460 if you can, it's better than the 5750/5770.
If you're willing to accept a bit less performance, you might find that a GT240, radeon 4670, 5650, might do. if you look, might find one of these in the ~~ 60$ range. You'll probably notice a performance drop, but it won't be like it's getting cut in half.
If you want to buy used just look on AT FSFT. Good deals on used cards everywhere!
Also, you might want to chance baking your card
