|OK. You've confirmed your graphics card & monitor are good.|
How are you determining that the system is POSTing?
Do you get a POST beep? If so then at least the CPU seems ok since without a functioning CPU you will get nothing.
If all you are getting is fans spinning and not much else you might have a faulty powersupply. PSUs provide several different voltage outputs on different rails. A failure on one rail can leave other devices getting power misleading you into thinking the unit is OK. Try another PSU.
Does the motherboard have onboard graphics or only the AGP card?
Goin' Fishin' (Some day)