mattpj's suggestions sound good to me, you might find that a 4300 is marginally easier to overclock due to the higher multiplier. If you don't want to overclock then you could go with the below specification and it is just over budget.
Asus P5N-E SLi nForce 650 (Socket 775) PCI-Express DDR2 Motherboard £69.99
(£82.24) £69.99
(£82.24)
Intel Core 2 DUO E6600 "LGA775 Conroe" 2.40GHz (1066FSB) - Retail £173.99
(£204.44) £173.99
(£204.44)
GeIL 2GB (2x1GB) PC6400C5 800MHz Value DDR2 Dual Channel Kit (GX22GB6400DC) £94.99
(£111.61) £94.99
(£111.61)
Sub Total : £338.97
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £8.25
Vat : £60.76
Total : £407.98