Don't get me wrong -- I am really pleased with how the card performs on its own at the moment. Just want to squeeze even more out of it.
I found this review and apparently the 6950 scales really well. http://www.techpowerup.com/reviews/ASUS/Radeon_HD_6950_CrossFire/
Normally, I wouldn't really consider it, but some one is offering it to me for £140 as opposed to the rrp of £225. And it's just really tempting.