You have GT SLi? If so I would OC those to 660-700|2000 and call it even. The GTX 280 will be better in some cases @ 1920x and up with AA active. Here's one glaring example-
Personally, I would not move from overclocked 8800GT SLi to a 9800GX2 unless you planned on going Quad SLi. I'd get a GTX 260 or 280 now, and SLi later if that is what you want to do.