A little more expensive than what you said but for £129.77 at www.microdirect.co.uk
you can get a 2500 barton (please note, not 2.5Ghz
) an Asrock K7S8X R3.0 and 512Mb DDR 400.
Your machine will run much faster with 512Mb rather than 256.
I've built a few machine's, similar spec, and they're quite fast.
options would be slighlty better asrock inc. Sata etc +£5
Geil branded memory +£5