Really basic, okay? RAID0 is two hard drives working in parallel, and they
must be identical drives. So if you have two 80GB drives, you see only
one 80GB drive as far as data storage, but it works like this. The odd
bytes are written to one drive while the even bytes are written to the
other, so you basically write twice as fast.
RAID1 is two hard drives, but they are mirrored. Whatever you write on
hd0 is written to hd1 so you have a redundant backup. Supposedly only
one drive would fail at a time, so if it did, you always have a backup.
RAID0+1 is like simple math. You have RAID0 and RAID1, so for this setup
you need 4 identical hard drives. You have a stripped (RAID0) situation
happening that is getting mirrored (RAID1).
To throw a monkey wrench in the wheel, AFAIK Linux cannot use hardware
RAID yet, as can Windoze. So you'll have to setup software RAID.
And before you pick your distro, you need to be certain it has a kernel with
support for SATA. Which means 2.4.27 or a 2.6.x kernel IIRC.
To answer another question, you cannot have one OS on one drive and
another OS on another drive in a RAID array. But that should be obvious
if you've read this far.
I'm sure if you search Google you'll find it explained and laid out much
better than I've done. I haven't ever read a manual, but I've run RAID
arrays in the past. And there are more than those 3 RAID scenarios.
Fail to read the below at your peril
Jeff Garzik's sata pages