looks like you have a cap ayam psu, that is always not recommended.
at least get a branded psu like cooler master, or even good brand like silverstone, corsair.

about the wattage needed, you can use psu calculator here: http://extreme.outervision.com/psucalculatorlite.jsp
should give you an idea..

haha, like bro lhongwuh say, ati card usually is cheaper when compare with it same performance nvidia card..

if you scare heat problem, get HIS IceQ cooler ati card, good cooling..