Are you sure it's just one +12 volt rail? It seems very week for a 500 watter. Make sure you don't have a combined +12 volt rating on the PSU, otherwise, the 8800 is probably suffering and your PSU needs a change.
Your card draws about 1 amp more than mine. The psu youre looking at doesnt have alot of power per rail, meaning that 18 amps may work, but youll be hampered for any upgrades. Look for something with 20 amps on each 12 volt rail. I know mine just barely gets by, which is equivolent at 18 amps per rail. Your card draws more power so youll need more than 18 amps. Go with a enermax or pc and c or something more upper tier
Make sure you know the combined amperage with more than one rail. 18a on rail A and 18a on rail B does not equal 36 combined. Probably more like 25a to 28a. If I remember correctly, EVGA recommends 28 amps for your superclocked? It's a good thing to have head room also. Try to find a PSU with 30a or more combined or single railed to be secure.