I'm thinking of going the whole way and doing the following upgrade... may even add in a Samsung Pebble 20" widescreen monitor.
Gigabyte GA P35C-DS3R, iP35 Express, S775 - £68.96
Intel Core 2 Duo E6550, S775, 2.33 GHz, 1333MHz FSB - £105.74
2GB (2x1GB) Corsair TwinX XMS2, CAS 4-4-4-12 - £40.70
512MB Asus 8800GT PCI-E 2.0 (x16), 1800 MHz, GDDR3, GPU 600 MHz - £173.77
500w Seasonic S12II-500
PSU, 12Cm fan, active PFC RoHS +80% - £53.69
Microsoft Windows Vista Home Premium 32Bit 1Pk OEM (DVD) - £60.97
£516.33