I quite like my ghetto ass AMD quad core. It's an.. x4 620 or something. Very cheap, decent performance, free up the cash to spend on a graphics card.
This. Cheap, cheerful and often has DDR3 support, so you can salvage your RAM if you upgrade later

PS. I dunno if Cheez meant a pre-built system, if so, look at these base units:
http://www.novatech.co.uk/novatech/barebones.htmlYou'd need to add a hard disk, CD / DVD and a decent graphics card. The rest is pre-installed.
There is an AMD II X4 630 Quad Core for £232.99 inc vat - easily within budget when allowing some extra for a hard drive, DVD burner and a budget graphics card. Obviously doesn't include Windows though
