Um, it's critical to find out if your power supply can support a new video card. If you have an OEM (Dell, HP, etc.) there's a good chance you're going to have to change out the power supply as well.
That said, Tigerdirect has some good cards on rebate for about $35-$50. A GT430 is a good card if you are confined by a weak power supply. A 1Gb 9800GT is about $35 and is better card, but draws more power. If you had a good 500-600W PSU, they have a GTS250 on sale for, I think, $45.