Just google Power Supply Calculators.
get a decent 500w Antec PSU, and a cheap 8800GT would be your best bet. as they are great cards and wont be obsolete for another 2 yrs. you can get them for as cheap at $110 now days cuz the new 9800GT's are out.
I'd just get a 8800GT
Newegg.com - 8800GT
see that PNY is $110 with a game, then the rest are $119.00 that card is your best bet.. you could even be fine with a 450w psu