You'll need to overclock your cpu if you're considering either going crossfire or a faster single card. You may even find that when overclocked, your cpu will still bottleneck you in some instances.
If you still decide to upgrade your graphics, I would say your power supply would be fine for crossfire, though I think you'll be better off with a single card. Don't go for the 660ti unless you can get it for a very good price; it's not much faster at all than the 660, but costs somewhere between an extra £50 - £80.
Imo, go for a 7950, and don't leave it too long before moving up to a new mobo and cpu.