Well, if it turns on and dies before the end of the POST, here are a few possibilities off the top of my head.
1.) CPU may be severely overclocked. This should've been solved when you reset the CMOS, unless it was done wrong.
2.) The heat sink fan may have come unplugged. Alot of motherboards will shut down automatically if no fan is detected. You reseated the CPU and HSF already, but it never hurts to check again.
3.) There might be a short in your case. Make sure there is nothing conductive touching the motherboard, and that your motherboard is not touching the back of the case.
4.) Doubtful, but both of the PSUs you tried may not be able to supply enough to power on the computer. PSUs do go bad, so don't be surprised if one eventually burns out on you.
It's not the RAM or IDE devices, and none of the peripherals could possibly cause a problem like that.
I hate even bringing up the possibility of someone's computer being plain dead, but it is a bit old...
Hope for the best, prepare for the worst. Hope I helped.
NZXT Guardian Case with Thermaltake PurePower 420w
FIC AU13 motherboard
AMD Athlon XP 2600+ 333FSB (Barton) with Thermaltake Silent Boost HSF
WDC 80Gb 8Mb cache 7200rpm
512Mb Kingston Dual-channel PC3200
MCP-T SoundStorm audio (integrated)
ATi Radeon 9800 Pro (it owned, until x800 and 6800)
Logitech Z-680 5.1 speaker system (integrated Dolby Digital 5.1, DTS, Pro Logic II decoding)
Logitech Cordless Elite Duo keyboard & mouse
Saitek Cyborg Evo joystick
Windows XP Pro SP1 (dont. install. SP2.)