NVidia recommends around 26A on the 12V rail for it to run smoothly but according to this site
http://www.gpureview.com/show_cards....rd1=557&card2=
it only uses 8A. I would much rather just buy a new PSU and have peace of mind. You don't want to stress a PSU either, it's better to get too big of a one and not have it fully loaded all the time. You can probably find a nice one for $40.