A quick update:
In the end, I built my own Q8400 system, using a cheap MSI mobo with onboard Intel graphics, a reasonable Coolermaster case, and (currently) 2GB RAM with a couple more on their way from Crucial as I write this. I've gone with Ubuntu 10.04 64-bit, which detected all the hardware (including graphics effects on the video hardware), job done!
Currently trying to convert my old Linux HDD into a VirtualBox machine on my new hardware - something which really is not as simple as it should be - but with luck that will be done by the end of the day.
I think the cost price of the components came in at around £360+VAT so it's a good spec at a reasonable price, although not earth shattering. It boots from power on in just a few seconds (thanks largely to a pretty quick BIOS POST - Ubuntu has been getting pretty fast anyway and my old PC let it down with 20sec to get past the BIOS).
Hopefully the speed improvements won't all turn out to be down to having a clean system - I could have save £360 and re-installed to achieve that!