It's true that the guys at overclockers favour the XMS RAM it's often used in their overclocked bundles, is good stuff. I used that kingston RAM (albeit the genesis C7 version) in my bros rig. It OC'd fine and have had no issues, i'm not gonna complain for what I/he paid for it.
I did question the GPU too although on paper it looks good. The OP can easily google benchmarks to compare GPUs and find one to fit the budget if he wants something else. I just checked and the 5850 is no longer on offer unfortunately which is a shame.
We both know if that Antec PSU was still on offer at £50 stulid would have spec'd it
It's always a compromise. It's so easy to "justify" another £10 here and £5 there, before you know it we are looking at a £600 build not £500.
I look forward to hearing what the OP thinks and see what he "settles" on.