with watercooling block for £ 50 or 2 air cooled for £44
whatd u rekon?
seeing that they can run up 180 watts, running sli isn't a good idea for your system. but the two air cooled are cheaper.
really i don't think you'll find a 6770 or better or gtx 460 or better for the same price; esp if you sell the other 260 that you don't need.
the biggest draw back is its an old card and cannot support directX11. but at £44 and sell the other for £24; it might be a good deal IF i mean IF its a good upgrade from what you have now.