Thanks for the reply. I've been looking at the Sapphire Radeon HD 7870 2GB and the MSI R7870 Twin Frozr 2GD5/OC. Both only cost about £30 more than the 7850 I'd initially looked at. I'm edging towards the Sapphire at the moment, but from what I can tell there isn't a huge difference between them. Are there any reasons for choosing one over the other that I might have missed?
Yeah even the 430W model of that same PSU would be more than fine. Your realistic peak power consumption should be ~250W. You might go a touch over that if you run some stress tests. Just make sure the PSU you choose has enough connectors for the GPU.
Might be worth considering the Samsung 840 SSD, similar price but a newer model.