You may need to be a bit more specific about what you use it for… Do you need decent graphics? Or is a faster processor more important?
If you need the best Graphics for your £350, I'd Buy the following…
http://www.ebuyer.com/product/176488
http://www.ebuyer.com/product/173791
http://www.ebuyer.com/product/173705
If you just want a powerhouse Quad core machine, and don't mind inbuilt Graphics on the Motherboard, or are prepared to upgrade the Graphics later, get the following…
http://www.ebuyer.com/product/173629
Both blow your budget by a few quid, but who's counting! Either will last for years and be very powerful machines.
Of course you can save yourself £70 or therabouts if you've already got a copy of Windows!
