Question about Time Machine-What type of hard drive?

I just placed my preorder today for a family pack of Leopard. One thing I'm looking forward to is Time Machine.
I have two macbook pros for me and my fiancee, a Mac Mini, and sometime soon an iMac I don't store sensitive data on the Mac Mini or iMac so backups aren't a big concern to me.
My main concern is the two Macbook Pro's which we both depend on for work. I have a external firewire drive that I back up to approximately once a week and store in a fireproof save. I use SuperDuper so create a complete bootable mirror of them.
I also have an Airport Extreme Base Station with a Western Digital 500GB hard drive. I store some items on that such as 70GB directory of clip art and also other important temp files and files that we use among the 4 computers. We have around 300GB free. We also use Chronosync which nightly backs up important directories such as Safari Bookmarks, accounting data, documents, mail data files, etc.
Now with Time Machine I'm not sure how to change my plans. I plan to do the two backups which are stored in the safe but will Time Machine work with my Base Station Hard Drive? Both MacBook Pro's have around 100GB of data on them and I'm confused if it would need 100GB to back up or less or more.
Am I better off getting a firewire drive that has to start on each MacBook Pro for it to work or will it work with a networked drive?
If networked am I better off getting a 2nd dedicated base station drive or is a pure ethernet hard drive faster in speed?

Seeing as Time Machine is not out yet, sounds like you know more than most. I guess we'll have to wait until Leopard's release before questions can be answered. I asked about Time Machine in the Apple Store and they said they haven't had training on it yet.

I don't have an answer, but I have a question along similar lines:

I'll most likely be getting a Mac Pro tower, and installing an additional internal HD to use as my backup. Most everything I've read about Time Machine to date mentions using external hard drives, but is there any reason it wouldn't work on an internal drive?

The only restriction I've heard about Time Machine so far is that it needs its own dedicated volume (i.e., not merely a folder).

Thanks...

It says on their site that you can use both internal and external.

I really wish it could use a folder, now I have to buy an entire HDD again...