Thanks for your help Stu, unfortunately I'm an indecisive git and think I might be better off doing a nice upgrade if I'm going to have to do a full reinstall anyway.
I'd been thinking of a Core 2 Duo, probably an E8400, which scan have for just over £100, or maybe I'll throw in an extra £20 and get a E8500.
It looks like I can pick up 2gig of Corsair memory for about £20-£25 (seriously, I paid about that for a 4meg stick many years ago).
So, yeah, I'd probably be looking at spending around £80 on a motherboard. I seem to remember someone recommending Gigabyte boards before (maybe earlier in this thread?), so I've been looking at those too. In fact it looks like the one you mentioned is on scan for £83, so that'll do nicely (although I'll have a proper shop around to see if I can find a better price).
I do have one question though, it seems that this upgrade will drag me kicking and screaming into the 21st century.
Every single new motherboard seems to have only 1 IDE channel, and I currently have a hard drive (on it's own channel) and a CDRW and DVDRW on the other one.
I could ditch the CDRW, but the IDE drive is currently my system drive. Is having it on the same channel as my DVDRW a bad idea? If I ditch it and try installing XP to a SATA drive is that going to give me a headache?
Edit - I should point out that I only now have a specific motherboard model to look at, so I haven't checked to see if it's compatible with the cpu I've pencilled in. Also, I'll check out the ram on crucial or something first, because I wouldn't want to do something stupid like order the wrong ram (I've definitely never done that before
)