I don't know what you mean by share it. You cannot connect it to both computers at the same time if that's what you mean. Aside from that, yes you can share it. You can access it over file sharing or swap it from one computer to another.
As for it speeding up your iBook, I would imagine it would due to the faster HD speed. However you may not notice much difference unless you are using the drive as a scratch disk for, say, photoshop or final cut. I don't know if running the FW drive as the boot drive has any performance advantage over the stock iBook.
Edit:
I have a very similar setup:
iBook G3 900, 640MB ram, 40 gig
Dual 2.5 G5, 3GB ram, 250 gig hdd
Dual 450 G4, 1.5GB ram, 120+160+40gig hdd
I use my iBook mostly for internet, word processing, and web design when I am on the go. I have, on several occasions, done 16-track mobile recording on it using a firewire drive. So, in this application the external drive really paid off. I would not have been able to do (trust) this type of recording without a firewire drive.