Not exactly the help I was looking for
Yup, but sometimes the answer to the question you asked is less useful than an alternate answer like this one.
In this case, it's wise to keep a minimum of two completely separate backups. I've seen it happen time and again that someone assumed they were safe with one single backup, and only discovered that that backup was damaged when they really needed it.
What you will want to do is get a drive that is at least 2-3 times larger than what you plan to back up. (If you just have the one internal hard drive on the iMac, and no data anywhere else, then it needs to be 2-3 times the used space on that internal hard drive, maybe plus a little more if you think that used space might grow a lot in the near future.) Then use that drive with Time Machine, and only with Time Machine. Don't try to put anything else on it.
Keep the drive with the Backup backups until you've got a good Time Machine backup going. Once that's done, you can decide whether you want to keep those old backups for a while or if you can get rid of them. If the latter, erase that hard drive and then start using it for a secondary backup. You could use Time Machine for a secondary backup as well, but I recommend using a different program altogether for the secondary backup. I like Carbon Copy Cloner, personally.
If you do want to keep the old backups for a while, buy yourself a third hard drive and use that for a secondary backup.
My personal backup strategy now includes four separate hard drives. One in a Time Capsule, using Time Machine, so I stay backed up on an hourly basis while at home. Another, also using Time Machine that I connect directly now and then. Then I have two drives that I use Carbon Copy Cloner to back up to. One of those drives is currently in the safe deposit box at the bank. The other is at home, where I back up to it. Every now and then, I swap them. That way, there's always one reasonably recent backup in a safe location, so that if something terrible happens and I lose everything in my home, I don't lose all the priceless data I've accumulated over the years (like digital photos of my kids, for example).
Thanks so much, and to gibbyice too, for the helpful replies. Thomas you have wise words here. I am getting a 2TB external tomorrow! I just got a new (well, used, but barely) Mac Mini server with two 500 gb drives. I want a clean start, and a better backup routine, when I start using that machine--just waiting on my new 27 inch NEC monitor!