Crossfire is for geeks who like playing with toys. It's not really practical unless you like benchmarking for a hobby. Most people would be better off buying a single GTX/ULTRA than two 2900's or GTS's. Single cards are more stable, provide better overclocking headroom and use less power.
Give me the several hundred pounds to get a GTX then. I've had a X1900XT for a while so adding the second isn't too expensive, and the dual card beats 8800GTS.