If you are positive that power supply works you have in there....
Start with the cd/dvd drives and disconnect one at a time until you get it to boot. If you get down to just vid card, sound card (if there is one), hard drive and ram; then take out one ram stick at a time, and then swap out the one and try the other; then remove them and then the sound card, and finally the vid card. If you remove the vid card and it boots, you have the solution, if not, get ready to replace the cpu or mobo or both.
My system: (updated)
AMD 64 X2 4400+ (Fast!)
Asus A8N32 SLI Deluxe
2gigs OCZ DDR400/PC 3200 (Fast!)
WD Raptor 150gb SATA 10,000rpm
WD 250gig X 2 RAID 0
Thermaltake Tai-Chi case with W/C
BFG Tech Nvidia 7800GTX X 2 SLI\'d
<b>\"If everyone is thinking alike, then someone isn\'t thinking.\"</b> --General George S. Patton