Gregg,
Quote:
The old drive is 120GB and already partitioned.
For your consideration #1:
Is a 120 GB external hard drive large enough for your use with Time Machine? A general rule of thumb is difficult to pin down.

For your review:

The What we think section in Inside Leopard: Time Machine

The Think big section in Is Time Machine all you need?

And Time Machine -- Frequently Asked Questions

For your consideration #2:
In addition to the size of the old drive from the G3, you might also think about the age of that drive and the number of years it has served you well. How much more time might it have left before some mechanical failure occurs and all of your Time Machine backups are lost?

Back up everything you can't afford to lose: documents, mail, movies, music, photos, and other data and settings.