OS 10.5 - Question about using Time Machine with AFP Drives

I am upgrading to Leopard today and will be using Time Machine on two MacBook Pro's.

If I'm right I can share one hard drive both computers, just separate folders. Is that true?

I think the option I hope to use instead of getting a direct connect firewire/usb drive I plan to get a AFP 'network drive'. I have heard if you want to back up across a network you need to use these, not drives connected to a Airport Base Station. Of the people that hvae chose this option, is it reliable and does it work well? Any suggestions of which drive to get?

My other question is, if I do this I plan to try to buy a Network drive that has a usb and/or firewire connection also. By doing this, will it be possible for example if I go on an important business trip to remove the 'network' Time Machine drive, take it on the trip with me to direct connect it via usb/firewire and continue using it and then revert back to network when at home. I know it's the same drive but with the route to it being different will it still work?

If not, another option I've thought of. I know there are a lot of .mac and sharing computer across the internet options in Leopard. Would I be able to back up to Time Machine across the internet while out of town so that I have my drive still backing up back Home on the network?