Hi there Drakken,
I know a friend who won an alienware rig, and it wasn't really all that great and so sold it to my other friend who only wanted it for WoW. As AW are part of Dell now, they really aren't what they used to be. With £3500 you could build a beast of a PC yourself, but it'd only last maybe 12 more than what an £850 rig would...
Here is a picture of the stuff i specced up for my PC with lots of help from the experienced guys here, most of them experts in one or more areas.
I haven't got it yet cause i was screwed over by a finance company who I've dealt with a few times before, will be waiting a few weeks til I have the money. But with your kind of spending power you could sort yourself out with something like that up there, upgrade a few of the parts. You wont really get much bang for your buck over £850 for a rig in comparison to that one, also thats overclockable to nearly as high as the Alienware rig.
If you want to upgrade some parts, you could certainly whack it up to 8GB RAM, Stick a better graphics card in, throw an extra optical drive in, take out the HD in favour of a couple of raptors, supersize the monitor and treat yourself to some pretty 5.1 speakers and a coolermaster case. Upgrade that lot will cost you probably another £800 but you'll have a bigger e-wang, and a slight performance boost.
Tbh tho, i have to say, if you have that much money to spend on a rig... Loan me £800 for mine ^_^