There simply is no point in purchasing a new PSU. Given the information, it does not seem to be the problem.
3.5" IDE cables do not provide power to the Hard Drive, the 4 pin molex power cable from the PSU does. Patchiz has stated that, when all the IDE cables are disconnected, and all drives are connected to the PSU, all hardware receives power.
Connecting the IDE cables will not increase power demand from the PSU.
Some more information that is needed. Patchiz, when you disconnect all the IDE cables, but leave all your drives connected to the power supply, does your system power on and POST successfully? (does the BIOS initialize and does the system try and boot into an OS?).
Also, how many Watts is your PSU?
Have you tried using a different IDE cable? And have you tried removing all IDE cables and only connecting one drive up to the motherboard with one IDE cable (such as the hard drive)?