If you have one already, have you considered a motherboard bundle?
Would save you a bunch of cash, if you have a decent PSU/HDD/DVD drive.
I ordered a barebones from Novatech when I wanted to upgrade, and reused my HDD, DVD and video card, and I was quite happy.
A barebones, is a case, PSU, mobo, CPU and memory, all fitted and tested before it's shipped.
You just take the lid off, plug in yer old dvd and HDD, and maybe a new video card, and it's all go
Not sure how this looks, it's a bit above budget, as you'll want to add a new video card, but all looks quite nice for the money.
http://www.novatech.co.uk/products/barebones/bb-37704d.html
I admit I've not compared it to other posts here, but I've used Novatech and they seem a decent bunch with common sense applied to their returns and warranty dept.
Had a piece blow on me, and talked to them, and after realising I have a clue, offered to take the broken card and courier me a new one, instead of having me send in the whole PC.