UPDATED LIST!
CPU:
http://www.overclockers.co.uk/showproduct.php?prodid=CP-367-IN&groupid=701&catid=6&subcat=
Updated! You dont completly need a i5 for gaming, but its nice.
Case:
http://www.overclockers.co.uk/showproduct.php?prodid=CA-063-CS&groupid=701&catid=7&subcat=1489 Updated! Great case, and 50 pounds only. By far the best case over the one you chose and vista.
Power:
http://www.overclockers.co.uk/showproduct.php?prodid=CA-055-OC&groupid=701&catid=123&subcat=1098 Updated! Not to much headroom for upgrades, but it is enough for now and a couple small upgrades, like a additional hard drive, or better cpu, or slightly better gpu.
Motherboard:
http://www.overclockers.co.uk/showproduct.php?prodid=MB-483-AS UPDATED! Very simaler to the old choice, just slightly older and less features that dont matter to much unless you want to use more than 1 gpu.
Ram:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-098-KS Updated! Highly reccomended by motherboards.org
HDD:
http://www.overclockers.co.uk/showproduct.php?prodid=HD-252-SE Updated! Smaller, but if you need the extra 15 pounds, here.
GPU:
http://www.overclockers.co.uk/showpr...odid=GX-299-SP Awesome GPU. Or, if you really need the money, you wont have a beast any more:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-168-MS
OS:
http://www.overclockers.co.uk/showpr...odid=SW-127-MS Windows, compatible with all games give or take 5.
Total 642 pounds or 562 with lower GPU.