It really depends on what games\apps you are using, I have machines that have both and tbh with what I run (WoW) I don't really notice any difference between the 7600GT, 7900GTX or 8800GTX SLI rigs that I run (admittedly the SLI rig runs a 24" monitor at 1920x1200 with ease where as the other two run on a 19" at 1280x1024).
If you're running games that require the horsepower then go for a 9800GTX (the same price as an 8800gtx now).
Oh and as an FYI the 8800GTX and Ultra's also run tri sli assuming you have the board to do so.
