Your PSU should be fine if you are only going with one graphics card, but i'm not sure that 33A will be enough for SLI. The 8800gt needs 22A per card, that's 44A total, and your PSU only has 33. So I would either get the one card now, and upgrade the PSU/extra 8800gt later, or buy them both now.
//EDIT here is a good PSU for SLI, the same company as the one you have, just more beefy, and enough amps to handle SLI:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3438609&Sku=C13-2503