I know people probly get tired of answering dumb peoples questions but if you could help me out id appreciate it alot...

okay, i have a dell dimension 4600 with 1 GB of memory and a Geforce 5200. I just recently bought an Nvidia Geforce 7800 GS OC to update. I also had to buy a new PSU (500 watts). Though, i just realized that the one i bought doesn't meet the requirements of (12V current rating of 20A or more). When i install the graphics card into my computer my screen is blank. (black) and i get no video at all. Ive read forums where other people had the same problem and everyone was saying it was because of the psu. IS IT? If i go through the trouble of returning and buying a new psu will it FINALLY work? THANKS IN ADVANCE.