|"Try swapping out the psu if you have a spare."|
The old PSU was a 250W, I think. I swapped it with my 450W and had the same result.
"Does it have onboard video? Use it."
That was the first thing I tried. No display. I swapped the graphics card. Same result. And it wasn't fried when tied with the old PSU, thankfully.
"Disconnect both the power & data cables ... Disconnect all external devices ... Now try to boot up."
I stripped this baby down to the bone, to the point where it was only the PSU, the mobo, the CPU, and the CMOS battery. I only did this to try and get a beep or two for not having any RAM. From there, I decided to at least try and slowly attach devices back on, from the RAM to the hard drives to the DVD-ROMs. Nothing. I'm about to give up and strip it for parts, it has a 120GB hard drive I can use for backups (although IDE, ugh)