Indeed. A 5XXX will be better. A 5850 if you can/want to pay for it, a 5770 if not.
And do you and your wallet a favor and get cheaper RAM. Dominator RAM (or any other premium RAM for that matter) do NOT offer even a single percent of performance gain over cheaper DDR3.
It's only about £10 saved, but these £10 could be spent on something worthwhile.
750W is far more than you need. A 500-600W PSU will be plenty, even enough for another crossfire down the road.
Are you going to overclock? If yes, you'll need a CPU cooler, if no, there's no reason to get a deluxe mobo.
By doing the above, you're already close to bridging the price gap with an ATI 5850. That's gonna yield you a massive performance boost and lower heat/power consumption.