Ok. I am a full time tech and I tell you this for one reason...I see hundreds of computer problems. When I see a computer come in that powers on and everything works, but no post beep, and no video, 9 out of ten times it is bad cpu. I would put a hundred bucks on bad cpu. On the other hand I have seen an occasional bad mobo out of this same scenario, or both. Yeah and usually there will be no post beep when this happens. Are you getting a post beep? Ram will do the same except typically gives a long post beep every few seconds when powered up.
Unplug all hard drives and opticals, cd-rom, dvd's, etc. then try to power on. Also remove add-on cards, pci etc. Process of elimination, Hope this helps.