How about the asrock dual sata around £50 and it's got a PCI-e slot and an AGP slot so you can stick with your current GFX card, and spend your budget on the following
CA-004-SK FSP Sparkle FSP400-60 Blue Storm 400W ATX2.0 PSU (CA-004-SK)
£39.95 £39.95
Asrock Dual Sata - £52 inc
AMD Athlon 64 3700+ San Diego 90nm (Socket 939) - Retail (ADA3700BNBOX) (CP-121-AM) £152.69 Including VAT
Don't know if PSU price includes VAT as that was a copy and paste
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But that brings you in or around £250 with a very good system, then you can upgrade the GFX at a later date.