Ebuyer is selling the Asus CM6730 desktop PC (otherwise known as the CM6730-UK010S) for £349 which is a more than decent price for a computer that packs an Ivy Bridge processor.
That particular PC is powered by an Intel Core i5-3330 processor clocked at 3GHz with 6MB cache and four cores. The processor can also overclock at 3.2GHz.
Other features include 8GB of DDR3 1333MHz RAM, 1TB hard disk drive (7200RPM), a DVD writer, Windows 8 OS, Intel HD graphics, keyboard and mouse, Wired and Wireless LAN, six USB 3.0 ports, HDMI and VGA connectors, a card reader and one 5.25in front facing bay.
The computer appears to have some decent expansion capabilities and cooling vents. Possible competition at this price includes the Zoostorm 7873 and the slightly more expensive (but also more compact) Acer Aspire XC600.
Interestingly enough, you can get a Zoostorm laptop with a Core i5 processor, a 1TB hard disk drive and twice the amount of system memory for £400.