I have 2 X 8800 GTX's that I had in SLI and overclocked before I got my 2 X 480 GTX's. I could overclock them to a bit past the Ultra specs without much problem. I did however have aftermarket coolers on them both. That is the only problem you might run into. They now make great dust collectors for when I need a test card.
They use a different GPU to the 8800GT and can't really be compared when it comes to OC'ing because of that.