The lowest I think you should spend on a GPU solution is $159, the price of the 2900Pro. If you are in a really low budget, the only sub $150 card I'd get is the 2900GT. The 8600GT doesn't stand a chance against the 2900GT, and it's only about $15 dollars less.
$115-125 get the 2900GT
$159-180 get either the 2900Pro or the 3850, depending on your PSU
$220-250 get either the 8800GT or the 3870.
Check out my other forum post for a good price on the GT. http://www.tomshardware.com/forum/247956-33-guys-later-round