Well, something else to check is whether or not your optical drives are
Serial ATA or
IDE. Each has its own connector and power plug. If your opticals are SATA and your hard drive is IDE, you won't be able to use their conenctors to put the hard drive in, and vice versa.
But if they're both IDE, which is somewhat likely, the power connector you'll use is the same 4-pin molex connector that would have been powering the optical drives before.