HD5870 vs 6870 vs 6950? Thanks

Hello everyone hope that you are all well and feeling the christmas cheer!  

I am upgrading my monitor to 1980 x 1080 23 inch LG one   and also I'm going to treat my self to an upgrade from the HD5770 which I am going to sell for about £90.

I really am not sure about which graphics cards are better than others with the new 6series. I know that HD 5870 is a beast though! I found an XFX one for about £200 from overclockers.co.uk, but are either the HD6870 or 6950 better? my budget is around £200 give or take a little. I do have the possiblity to crossfire on my motherboard as it has 8x/8x pci express slots, but I would have to upgrade my PSU before I did, but It it defiantly a future consideration for an upgrade  

Please let me know of the pro's and con's of each card and what one out of the three is the best ( or any other card out there ) I do however have a parts preference of ATI. I will be purchasing this card before the year is out.

Thanks for your help

(P.s my system specs are in my sig, cheers   )