Looks very nice, with an HD 4850. Also change the motherboard to Asus P5Q Pro. For 9 pounds more, you get the ability to add a second HD 4850 later. Your PSU and case are already perfect for that.
http://www.ebuyer.com/product/145751
The HD 4850 you found there is £150 and has 1GB of RAM. On a 20" or 22" monitor the extra RAM doesn't really do enough to be worth the extra cost, IMO. On a 24" or larger it is worth it.
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