please tell me what game you are playing that makes you think your single 8800 ultra is not enough..........If I were you i'd wait a year or so ... it will be that much closer to being practical then..... also cheaper.
What resolution do you play your games at? If it's anything in the 1xxx by 1xxx range, then you most likely wont even see a difference, since your single Ultra card can run them fine. Now if you're running resolutions in the 2xxx range, you might benefit from it.