This is probably the best upgrade.
PCPartPicker part list: http://uk.pcpartpicker.com/p/v1fh
Price breakdown by merchant: http://uk.pcpartpicker.com/p/v1fh/by_merchant/
CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor (£73.30 @ Amazon UK)
Motherboard: Gigabyte GA-970A-D3 ATX AM3+ Motherboard (£67.04 @ Dabs)
Memory: Crucial Ballistix sport 4GB (1 x 4GB) DDR3-1600 Memory (£15.20 @ Amazon UK)
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-02-22 11:58 GMT+0000)