Crossfire just means using 2 of the same cards in 1 setup.
With the 660s, there is 2 versions, the normal 660 and the 660ti.
The 660ti is the faster card
With that resolution 7870 is fine, I ordered a new system with that card. I can't wait
Well the £250 budget is because I'm switching from consoles to PC, I noticed how terrible consoles were and they only get the most attention from gaming companies since the mass amount of money is there, so since I'm switching I'm selling my xbox, PS3 and a load of games. (Like 60)