A 8800Gt will cost you $136? Man, can't you get one on eBay for like $50? $136 will get you a single 5770 512MB.
Here is a link to a Tom's chart that compares both setups. The (2) 8800 GT's in SLI gives a single 5850 a good run. In most cases it comes up a little short or slightly behind, in a few it beats it out slightly.