When the monitor lights up for a few seconds... Do you see anything on it ?
Any beeps ? (do you get the initial one beep, or any BIOS error codes ?).
First, I'd strip it down again (no IDE/SATA/PCI devices).
Then, try booting without RAM, with only a single RAM module, and/or wihtout Video Card to see if you're getting any BIOS error codes (one long two short usually).
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