If you have a Rev A PowerMac, you can add that second drive. There's not much in it between the different manufacturers these days, so my preference is that if you're maintaining the existing drive, it's better to have your second drive be by the same manufacturer.
Obviously, this isn't actually true. In the PATA days, it could make a difference, but each SATA channel is separate so it makes no difference - it's just a silly tradition, I guess. 🙂
If you have an iMac G5, then you'll be replacing your primary drive. You can't add a second drive. You'll want to borrow an external drive enclosure so you can copy your drive contents across to your new drive.