I'm delighted to report that my Mac is back, and better than before! The faulty logic board has been fixed, and everything is hunky dorey now. It was repaired for free under a Repair Extension Program, which is a good job as a new logic board costs £350, plus labour to install. Its such an important component (I knew it as a motherboard), and after hammering my Mac with loads of downloading files and video editing for two solid years, it's a bit of a bonus to have it replaced for free. Glass half full, that's me.
I did a clean build of Tiger, and oddly enough I have freed up about 40gb of space, even though I've put all my old data back on the machine. I guess Mac OS X doesn't clean up old files the way it should.
Anyway, if you're curious to see what happens when your logic board gets fried, then I took a few screen photos which can be seen here.