It depends on your price range. If your looking into the lower end graphics cards that have a minium of 2gb of vram i would look into getting the relitively new AMD 7850. If you would prefer to be upgrading to NVidia then i would get a 560Ti with 2gb of vram (Be careful because NVidia released 2 560ti cards: 1 with 1gb of vram and another with 2 gb of vram.) Both of these cards can be picked up at around the £200 mark (around $310)
If you are willing to push above the £200 price range and start looking into the highest end graphics cards i would definately recormend the GTX 690; this card is massively expensive but in my eyes worth the upgrade. i got one of these cards because its goig to last a long time before it needs tot be...