Weird - never seen RAM do that before - that is, actually die while the system was up.

Found out it was the RAM by connecting an old IDE drive with (by chance) Win98 on it. It wouldn't boot with it , just whinged about Divide by Zero errors. But I then booted it up in safe mode, and it said it couldn't load the XMS memory driver... due to dodgy RAM.

As luck would have it, my PC uses the same RAM as the Mac Mini, so I swapped out the 512 stick from the PC, and put in the 256 that originally came with my Mac Mini. Lo and behold, it boots happily and all's well. (Except for it living on 256 MB RAM til I get some more...)