Okay, I've gotten irritated enough with this to find a forum and post here.
The initial machine:
1Gb Corsair XMS
Radeon 9600 (Sapphire)
2x SATA HDD
2X PATA HDD
Thermaltake Xaser III case
Soundblaster Audigy2 ZS
Buffalo Wifi card
I ran it for about a week straight with no crashes, restarted and it got on to the POST screen, started writing the RAM test text to the screen and stopped dead.
I reduced it to the PSU, Mobo, CPU, GFX card and RAM and still the same.
I replaced the GFX card with a Radeon 7000, still the same.
I tried the RAM modules (2x 512Mb) in various combinations - same.
I tried another different RAM module - same.
I also replaced the Mobo with an IS7-E with a P4 3Ghz - same.
Changed the PSU with another - a 400W PSU - same.
After all that, I'm not sure what could be the problem - unless the original PSU fried BOTH the mobos to some extent, or the CASE is causing the crashes.
Any here have any ideas of what exactly is going on here?