To stick to you budget I would advocate a 6800LE or a 9800 pro. A lot of discussions here on this forum about 6600GTs (sorry you will have to do the search) but they seem very power hungry and do a similar job to a 6800LE
herc182, the 6600GT is power hungry and a 300w psu wont be adequate. The manufacturer XFX now state you need a psu rated at 450w as a minimum. There were a lot of people who were originally caught out by the power requirements of this card(myself included unfortunately), so if you go for the 6600GT, a new psu would be required.
Bare in mind that the 6600GT needs to be connected directly to your power supply.
click here for a slightly cheaper 6600GT than the one you found with lots of glowing reviews.
garrema's advice makes sense if you don't want to go to the expense of buying a more powerful PSU.