For gaming an AMD hexacore is not good value for money as it performs the same as the ~£130 Phenom II X4 955. Often people faced with this argument come back and say that they want a bit of futureproofing, which potentially a hexacore would be able to provide over a quad core.
The best performing/priced PSU in Britain is the XFX 650W on ebuyer @ £70. It's more than you'll need for one 5870, but not enough for two, but it's my default suggestion and you don't appear to be interested in crossfiring anyway.
The only 'advantage' I can think of to Win 7 Pro is that it has XP mode, which won't be that useful to most people. However if you can get any edition for £30, might as well get the highest end one you can.
Thanks for all the replies, but I wonder if I've almost set my budget too high. GTX470 CF certainly sounds to me at least overkill for a single monitor set-up, probably going to be around 24-26". Would it be worth going for an Core i7 Quad, and putting a bit less into GPU's, with a mind for CF/Card upgrade in the future?
Yea I don't see the need to get two 470s at the start of a build unless you're thinking about 2560x1600 kinda res.
An i7 920/930 is also a bit more than you strictly need for a gaming build, but it would maintain a high performance level for quite a long time.