Boot Disk

What is the best to use as a boot volume, when you have four bays to choose from, and you have to replace one which is really causing severe delays to access?

80Gb, 320Gb, 640Gb, 1Tb. 1.5Tb, or 2Gb? SATA or SATA II? External? What's B grade OEM?
Is bigger slow, or is it OK to keep all you can fit on one drive, and get more per bay?

Do think SSD would be reliable as a boot, or does it have an annoying type of fragmentation...?

What's a network startup? NAS or SAN?

A Mercury Extreme SSD from OWC works great as boot disc for Snow Leopard in position one on a Mac Pro. Boot time is less than eighteen seconds. Disk two is a 10,000RPM Raptor disk used as backup and Bootcamp drive, and it is so s-l-o-w by comparison when booting.