The popular choice amongst GPU's, and I've recommended it to others who want the highest game settings or overclock their GPU, is to get a reference 6950 and unlock it to a 6970. They're about £200 on scan's website, just a little more than your 6850. And you'll also want to keep that 8GB or RAM. I can hit 4GB easily during BF:BC2. You're PSU is more than enough also.
Might possibly need about 750W though if you want to Crossfire 6950's unlocked to 6970's in the future. 6850's in crossfire will pull around 400W, 6970's in crossfire will pull about 550W. Those power measurements were in bare minimum setups also according to a TweakTown article. So, you're 650W PSU would be more than enough for crossfire 6850's but you'd might want a little more cushion if crossfiring the 6970's. When I refer to 6970's or 6950's unlocked to 6970's, take it as the same thing as the power consumptions would be identical between the versions.