Okay so for £50 over you could get:
CP-135-IN Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - OEM (CP-135-IN)
£109.95 £109.95
MB-061-GI Gigabyte GA_965P_DS3 (Socket 775) PCI-Express DDR2 Motherboard (MB-061-GI)
£89.95 £89.95
MY-058-GL GeIL 2GB (2x1GB) PC6400 800MHz Ultra Low Latency DDR2 Dual Channel Kit (GX22GB6400UDC) (MY-058-GL)
£107.95 £107.95
HS-017-AR Arctic Cooling Freezer 7 Pro CPU Cooler (Socket 775) (HS-017-AR)
£14.95 £14.95
Subtotal £322.80
VAT £56.49
Total £379.29
Or this:
MB-144-AS Asus M2N-SLi Deluxe (Socket AM2) PCI-Express DDR2 Motherboard (MB-144-AS)
£89.95 £89.95
CP-166-AM AMD Athlon 64 X2 Dual Core 3800+ 2.00GHz (Socket AM2) - Retail (CP-166-AM)
£89.95 £89.95
MY-034-GL GeIL 2GB (2x1GB) PC5300 667MHz Value DDR2 Dual Channel Kit (GX22GB5300DC) (MY-034-GL)
£99.95 £99.95
Subtotal £279.85
VAT £48.98
Total £328.83
Or if you wanted to keep your RAM and be under budget:
CP-127-AM AMD Athlon 64 X2 Dual Core 4400+ (Socket 939) - Retail (ADA4400CDBOX) (CP-127-AM)
£149.95 £149.95
MB-111-AS Asus A8N-SLi Premium nForce4 SLi (Socket 939) PCI-Express Motherboard (MB-111-AS)
£85.95 £85.95
Subtotal £235.90
VAT £41.29
Total £277.19
EDIT: I know the top one is way over but I'd forgotten a heatsink

However it will offer excellent overclocking potential and I *think* will reach E6600 speeds