On eBay, there's a similar machine for about £260 but it's not quite as good as yours. I'm not from the UK and this might be a bit ignorant, but I reckon you could charge a little more considering the specs of yours are better. 🙂
mad! to say the Mac Mini PPC is now obsolete it's going for a fair bit. Then again I've got a mate who wants to buy my perfect condition PowerBook for £700 (only taking a £200 loss). I'll probably sell for 500 anyways.
Then again consider you can get a new intel Mac Mini for £350 with a student discount...