£5000....blimey,you have money to burn.
I think you are seriously spolit for choice with that budget.You will have plenty of change.
Firstly think of your games and operating system.
Vista is reknown for being unable to play to play a lot of the older games,but if you want the DX 10 games,you need Vista.
Im not a gamer, but plenty here are and maybe you will be better advised by them.
My advice would be to ring Novatech and explain exactly what you want and let them build one for you.
They will advise you accordingly if you have a few issues in what to select.
Novatech are if not one of the most reliable companies in the country (uk)click here