You won't get as much as the maximum data transfer speed (DTS) would indicate. Each time you add a track to ProTools and simultaneously play them all back, the disk must seek to to retrieve each tracks next block. So unless you have of of those Velociraptor 10,000RPM drives with the very fast seek times, seeks dominate the performance calculations. Even FW800 doesn't help much of FW400.
You get the best performance (with FW400, 800, or eSata) by having several drives dedicated to ProTools audio, and using PT to round robin track assignment. Various forms of RAID actually slow down PT (according to Digi, and it makes sense).
I routinely do 32 tracks on my PT HD Accel system (MacPro Intel 2.66 quad) on a single FW800 drive. If I have a project that needs more than that, I'll round robin a second drive.
See the Digidesign site for information about disk requirements, maximum track counts, etc.
Eddie O