The ONE THING I HAD NOT BOTHERED TO REPLACE/TEST TURNS OUT TO BE THE CULPRIT. Turns out my 2x 512MB Corsair XMS are offically, for all intents and purposes, dead. I popped in 2x 512MB of Kingston generic value RAM, and it booted up fine. Had to reinstall XP after all of my hardware replacements, but I think that's a small price to pay. Just wish I hadn't accidentally wiped my system hdd heh.
Anyways, infinite thanks to everyone in this thread who contributed their ideas. Even though they didn't help me, I'm sure somewhere down the line they will help someone else.
AMD X2 4400+ @ 2.5Ghz | ASUS A8N-SLI | 2GB Corsair XMS DDR400 [SIZE=\"1\"](2-3-3-6)[/SIZE] | eVGA GeForce 7900GT [SIZE=\"1\"](500/1500)[/SIZE] | 1TB of SATA harddrives | SB X-Fi | Antec TruePower2.0 550W