Going to chip in and say you could for £500 get 2 x HD9750
just a quick look and the Gigabyte Windforce 3s are £239 at the moment at
overclockers (could probably get cheaper else where but it was a quick look)
I saw FSX was mentioned and although I'm not sure if it's that old surely switching off CF for it wouldn't harm your performance that badly? Forgive me if I'm wrong. Another bonus of getting these cards is the fact you get two versions of Crysis 3 and Bioshock to sell
making it cheaper.
If you're worried about performance I wouldn't something like Crysis 3 can run on these badboys @5760x1080 "medium system spec" but with Very High Texture detail at SMAAx1 with 40 - 60 FPS, I haven't tried any higher AA settings as it looks stunning at those settings.
Do take into account what the others say too as Crossfire is not for everyone