Thanks, mcthomas. I hope that, after you have some experience with SuperDuper!, you'll be chiming in with them, too!
You can partition your HD into as many partitions as you'd like. The rule of thumb, in general, is to have enough space to store what you think the *maximum* amount of data you're going to have on the drive is.
So, with an 80GB drive, if you think you'll never get over about 60GB, you'd need a 60GB partition to do a full backup. We won't span drives or partitions, though, so you'll have to make it large enough, or you'll need to repartition later (a program like iPartition can be used to do that with data in place, should that become necessary).
Remember, too, that if you're planing to mix data and backup on the same drive, isolate the data in its own partition -- don't try to combine the two.
Hope that helps!