Is there a performance issue you are trying to correct? Is it for benching? Do you just want to try out a crossfire set-up? If you are gaming at higher resolutions and the 8800GT is holding you up go ahead. Want higher benching number, that will do it. Just wanting to try out cross-fire? why not. Upgrading just for the sake of upgrading? I have been known to as well. Is that the most performance you can get for the dollar? Probably not.
In general I am not a fan of crossfire or SLI. If I was going to spend that kind of money I would probably either go with a single 3870X2, 2x3870Xs, or a 9800GTX2. If you go with a dual card set-up go with two high end cards, IMHO. But ATI has creating an interesting option of 2.5 high end cards as an option. If I had the cash and a resolution to justify it I would be playing with either quad crossfire or SLI, it is just to interesting to ignore. Heck if I had the cash I would as I don't game but I do like to get my hands on new and interesting technology.