You don't say what kind of mac it is; from the clock rate it's apparently fairly old.
First, I would check what kind of drive is currently installed. If you're using OSX this is easy, go to "About This Mac" and select "More Info". Step through the hardware till yo find your drive. My PowerBook's drive is found under "ATA" so obviously I have an ATA/IDE drive; yours is probably the same, but you'll want to check.
Assuming that it is, you should be able to put another IDE HD in it. If you have a PowerMac, it should have a second bay to install a second disk in. If it's an iMac, you'll need to have your operating system disks so you can reinstall the OS on the new drive.