Here's a list of best cards in each price range:
http://www.tomshardware.com/reviews/best-graphics-card,2569.html
For what you've described you'll need at least a Radeon 4670 (about $75) and preferably a card in the $85 range like a Radeon 5570 or GeForce GT240.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102855
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102874
http://www.newegg.com/Product/Product.aspx?Item=N82E16814133306
Performance gets better as prices rise, so you'd see a nice boost spending $100 for something like a GeForce 9800GT or Radeon 4770. However, cards in that performance tier and higher often require a power supply upgrade as well. Those two cards require a 400W power supply, while the Radeon 4850 and GTS 250 require 450W. Unfortunately most brand-name computers come standard with 300W power supplies.
It all depends upon how much your budget can accomodate.