internally, the zip drive bay should be the correct size for a hard drive, but you would want to make sure... so you will be able to physically mount a hard drive where your zip drive is, yes.
on your hard drive you have a ribbon coming off and going to your motherboard. it has 2 options:
Hard Drive > Super Drive > Motherboard
Hard Drive > Motherboard & Super Drive > Motherboard
the difference here is how it is routed... the ribbon has 2 plugs on it... but, most of the time, when you only have 2 drives, it is best for you to put them on separate ribbons (faster throughput i would guess).
if you do have 2 ribbons in your mac, and one goes to your hard drive, and one goes to your superdrive, you can easily attach another hard drive to the 2nd plug on the primary ribbon (the one with the master hard drive)
one thing you do want to be aware of though... im not sure how picky mac hardware is, but from my experience with pc's, the computer will only recognize the hard drive when the original hard drive is set to master, and the new one is set to slave (master is the one with the bootable OS on it)
uh... if you have any specific questions, just ask me... otherwise, i could get into more detail, but you wouldnt need to worry about it past what ive already said