Now to throw a spanner in the works
Have you considered a crossfire rig? Now hear me out before going no thats a stupid idea!
You can buy 2x 4850s from Scan for a touch over £200 and they will out perform the 4870 and the GTX280 (in almost every game, crysis is an exception as crossfire scaling is not the best, but it still out performs the 4870 here). For this to be at its best you will need a x38 or x48 motherboard and the DFI DK X38 only costs £105.
So what do you think? a decent 600W ish PSU (Corsair HX620 or TX650, Enermax Mod82+ or Pro82+) will be required but you were looking at that size anyway.
This is just an idea, but one worth of some consideration in my opinion. Crossfire has matured a lot since it was first touted and it just so happens that the 4850 is probably the first card that actually makes it worth while over a faster single card option because of the great price to performance this card offers.
If you fancy a bit more reading on it then this is a good review comparing all the 4850/70 and GTX260/280 options including SLI and CF
http://enthusiast.hardocp.com/articl...50aHVzaWFzdA==