For example, this Antec has 3 +12v rails of 18a each, is 500w total and is 90 bucks. Antec makes a good solid PSU and I would consider them one of the better brands.
I would consider that PSU the absolute minimum choice, but it may not be enough if you upgrade your CPU, or add more components or SLI your video card later.
For a little bit of future proofing you may want to go even bigger.
This one is twice the price but is 750w, premium brand, is certified for dual 8800GTX cards in SLI and has a good efficiency rating.