K m8 heres a few examples of what we were talking about , im assuming your budget at £750 but will include a couple under that price :-
Im also assuming you need a monitor - if you already have a monitor let nme know and ill re price without.
http://www.pcspecialist.co.uk/ - this is a site where you customise what you want , basically it gives you options on what to get , you want to be looking at at least 2gb ram , vid card = 3870 , a core 2 duo processer at 2.2ghz and a 600w psu (a good one not cheap unit) .
Also ive done an example for you , ive PM it to you because ive to give you my email address and a password for you to look at the saved quote and i dont want to be posing my email and stuff on website!
This is a good system - but with no monitor or keyboard/mouse so ive podted a monitor below it
http://www.aria.co.uk/Products/Systems/Gladiator+Titan/Gladiator+Titan+PRO+Extreme+PC+System+?productId=27004
http://www.aria.co.uk/Products/Displays/Monitors/TFT+19/AOC+22%22+Wide+LM22%28Esys%29+OEM+LCD+TFT+?productId=31327
Or (again add monitor/keyboard) http://www.ginger6.com/intel-ultra-gaming-p-29381.html
O a complete system - 3rd one down on the page http://www.autdirect.co.uk/acatalog/Intel_Systems.html
Or http://www.pc-warehouseuk.com/product_info.php?cPath=39_52&products_id=114 - this £100 under your budget, its a good system with Quad core processer, only downside is that in 18months you may need to be upgrading the graphics card but other than that its a good system for the money.
Any questions mate dont hesitate to ask , i can probably find more with a bit more time but i think the first one i posted is best value for money , depends on your own opinions as well though.