It sounds as if the motherboard doesn't pass the self test.
My experience is that it usually is a problem with the PSU.
RAM problems would give you some beep "error codes", and unless your HDD is burned out and short-circuited (which probably would burn your hdd controller(MB) as well), fiddling with the hdd cables, doesn't change a thing.
Hard-drive errors will in most cases bring you to some stage in the BIOS self test (generating at least some signal to the screen).
Try to find another power supply unit (PSU)and test if you can get it to boot with that.
good luck :)