No bios beep codes at all? Remove all memory, and if mobo is good, it should beep. Same with cpu.
I'd try to reset the cmos, unplug ang reseat all connections, reseat all cards and memory modules. Sometimes something just gets loose and it wont post. Long shot but at this point worth a try. Also make sure the cpu hs fan is working....might also see if its just the cpu, if you have access to to another chip of that format. Another easy enough thing to try is a stripped boot. Incidentally, I've seen cases where a fried modem would cause the same symtoms: remove all but video card. Disable all onboard peripherals in bios to diagnose they might not be causing some problem. Also recall a compuqe that wouldnt boot unless the reset button was hit after power switch, and another with a weird 20=pin board connector that needed wiggling.
Just be forewarned usually oem stuff like emachines, compuqe, etc you cannot replace the psu, case, or motherboards due to proprietary configurations. That said, I have modded a few cases and such to make it work, but often its fruitless.