You'd have to remove the optical drive to make it fit (running the machine half disassembled is NOT recommended!). It's doable though. You don't mention which model you have, if it's the slotloader there are two ports on the same cable and they definately are standard IDEs. Personally I used an external Firewire box when I did this though.
EDIT: I intially made another answer because I thought we were talking about a Powermac - we are in that forum, after all.