Antec 900 all the way.
By the way, I strongly suggest you get yourself a better CPU cooler. Assuming the stock ones can actually manage the enormous heat produced by those cores, it'll sound like a bloody jet engine. You can easily get a good one for less than £40. However, you may have a little trouble installing it if Gigabyte have been playing silly buggers and installed enormous capacitors around the CPU.
Otherwise, it's a very nice system.
(With regards to operating systems... I would have got XP x86 and Vista x64. That way, you'll pretty much guarantee that everything will work and not live in fear that MS won't arbitrarily drop support for your OS overnight. Use seperate hard drives, obviously. )
EDIT: @JDawg
Unfortunately, Newegg don't ship outside the US (they might do Canada, I'm not sure) but that isn't really a problem. Despite the fact that the dollar is getting pwned really, really hard by the pound, anyone who wants to order computer equipment is going to have to pay some obscene import duties (including VAT). It's much cheaper to simply order for us to order from UK companies (or fly over to the US and pick it all up*).
*I say that, but I don't fancy queueing all night outside the US Embassy in London for a Visa.
Edited by - The Evil Thing on 9/28/2007 1:26:53 AM