Wow thats a real eye opener right there, 100 quid difference between the intel and AMD spec, GPU currently is a ATI 5870, I have 2 options foor the GPU, add another 5870 in and run in crossfire or spec a new card, Its not critical straight away as im not a massive games player.
Your 5870s are heavily limited by the fact they have 1gb vram. That sort of vram was out dated as soon as Battlefield 3 launched.
However, given you're not a massive gamer then you'd find that two 5870s are bloody fast. I would use your AMD saved pennies to put to a GPU. Something like a 7950 would show you a much bigger gain than going Haswell and keeping your 5870.