|This smacks of bios/cmos problems to me. I once had a similar problem with a msi board, cleared the cmos repeatedly to no effect, until I removed all the ide cables then cleared the cmos and it worked.|
Try removing everything except the video card,cpu and ram then clear the cmos with the battery removed and the power supply disconnected and make sure you use the right jumper, give it a few minutes as some times the cap can be a little stubborn giving up it's charge even though it is meant to be shorted. Replace the battery, cmos jumper and power without connecting anything else and try to start it.
Since you have already screwed with the bios you may also need to recover it, asus seem quite paranoid about it a have a couple of methods to recover the bios, the bios venders have their own methods if the you want to try theirs instead http://www.bioschip.com/biosrecovery.html.
I belive http://support.asus.com/download/download.aspx?SLanguage=en-us&model=P4P800-X is the support page for your board so you can get bios, manual ect.