Here is my recommendation.
All the decent graphics cards are on pre-order at the moment, sorry about that.
I included Windows 7 Home Preimum as if you install XP you won't be able to use all your RAM, and I included a DVD drive because I don't recommend you use an IDE one. If your current one is SATA, then just remove it from the build.
Consider upgrading the PSU if you intend to upgrade it in the future to an HX 650, but they are also on pre-order.
On crap I forgot the cooler! Here, you'll want an something
like this. If you don't need a new DVD drive that should put you just under £900 with the HEXUS free shipping deal.