Choosing a new graphics card

hi, i need help choosioing a new graphics card, i have read about the sli and crossfire thread, and learned a few things. But am still wondering what is the best GPU i can get for my system, it supports crossfire, so that rules out the nvidea's, but which would you say is the best GPU/GPU's for around £100 below is my sytem configorations, so i hope one of you can help me choose a good one. I am running windows XP.

MOBO:
Asus maximus formula II p45 chipest

RAM
Kingston hyperX 4gb 1200mhz

Processor
Intel core 2 duo E8400 'wolfgang', 3.00ghz

Power Supply
x-clio 650W power supply