Have just completed a similar system for a good friend of mine, we looked at the following options:
AMD System:
AMD AM2 Athlon 64 4600 + Windsor Core, Dual Core 2.4GHz, 2x 512KB Cache, Energy Eff' Retail - £75.19
Gigabyte GA M61SME-S2, NF405, AM2, PCI-E (x16), DDR2 400/533/667/800, SATA II, ATX - £33.32
1GB CorsairXMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-12 - £45.40
Total - £160.94 inc p&p
Intel System
Intel Core 2 Duo E4300, Socket 775, 1.80 GHz, 800MHz FSB, Allendale Core, 2MB Cache, Retail - £73.17
Abit IB9 i965, S775, PCI-E (x16), DDR2 533/667/800, SATA II, ATX - £64.50
1GB CorsairXMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-12 - £45.40
Total - £190.09 inc p&p
Either add another 1GB later or swap 1GB for 2GB:
2GB (2x1GB) CorsairTwinX XMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-12 - £80.65
Or if that's still too expensive, change the ram for the cheaper:
1Gb Micron Major, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-12 - £31.71
Which will take about £15 off the price, or £20 if you go for 2GB.
256MB Sapphire Radeon X1950GT, PCI-E(x16), Mem 1200MHz GDDR3, GPU 500MHz, 36 Pipes, RoHS - £86.96
1GB AMD with X1950GT - £247.90
2GB AMD with X1950GT - £283.14
1GB Intel with X1950GT - £277.05
2GB Intel with X1950GT - £312.30
These prices are from scan.co.uk. You could also try microdirect.com, or ebuyer.com. overclockers.co.uk is another good site in the UK.
Just thought this might be of interest. When you do decide to buy, price up your system at each of the sites i mentioned, depending on the offers at the time you may well get it cheaper at any one of those sites.