Western Digital Caviar SE16 250GB 2500KS SATA-II 16MB Cache - OEM £42.99 (£50.51)
GeIL 2GB (2x1GB) PC6400C4 800MHz Ultra Low Latency DDR2 Dual Channel Kit (GX22GB6400UDC)
£114.99 (£135.11)
EVGA GeForce 8800 GTS KO ACS3 SILENT 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail
£289.99 (£340.74)Gigabyte GA_965P_DS3 (Socket 775) PCI-Express DDR2 Motherboard £69.99 (£82.24)
Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - OEM £101.99 (£119.84)

That is from www.overclockers.co.uk

While at £728.44(incl VAT) that's a bit over your current budget it does get you a much better CPU and motherboard and a silent cooled GTS as well as 2GB of RAM. Quite frankly right now anything other then a C2D processor is a waste of money. With that mobo the E6300 can be easily overclocked to over 3Ghz from its original 1.86Ghz without any extra cooling. To be safe you might want to spent £10 on an artic cooler freezer 7.

If your 6600GT is PCI-E then I'd stick with that for a few more months as the cheaper nVidia DX10 cards are just about to be released along with ATis rival cards which will be very interesting to see how they compete with the nVidia cards.

The DS3 and E6300 were born to be together and are the perfect overclocking setup. For £200 you can have one of the most powerful desktop processing solutions available.