1. SATA Raid kinda comes as standard, however they will run as single drives quite happly (i'm doing it currently..)
2. Works fine, I used to use them before I got proper SATA drives to convert my IDE drives.
2a. well, yes and no [grin] You are basicaly moving harddrives around, so while windows will still work you might have to reallocate drive letters to get things to work as they did (for example, if you move your D harddrive to SATA the change of adapter could well end up allocating your D drive to E or something). I'm not sure about moving your boot drive, but that might not work with Win2K/XP as the boot.ini stores the boot device in it and i'm not sure how that mapping is effected if you try to boot from SATA when it was on the primary IDE, my instincts tell me it'll muck up however...

3. Speed mostly. While atm you wont notice the effects once PCI-Express really gets going they should be able to ramp the drive speeds up to stupid levels. Also, the SATA cables are smaller (only 4 core) so less clutter in the PC = better air flow. There are probably other things but those are what sold it to me [smile]

There's no real speed difference.
Drive speeds aren't limited by the bus. (A good drive didn't even use the bandwidth available on IDE-100)

The reason disks are so slow is that they're relying on moving parts. You can only spin a disk so fast before bits of it falls off, tears through the case and breaks your leg (assuming the case is located on the floor, near your leg) ;)
It doesn't run over the PCI bus either (as far as I know), so PCI-Express won't change anything.

However, SATA isn't limited to one master and one slave drive per cable, like the old P-ATA bus did. And the slimmer cables does increase airflow. I think the cables can be longer too.

But mostly, its just a new standard with more bandwidth (even if that doesn't actually make a difference yet), with better designed cables (although they can be a bit fragile), and which supports a few other things like TCQ (or is it NCQ?) and I'm pretty sure the standard allows hotswapping drives as well.

yeah, the standard allows for hotswapping.

SATA drives do transfer data faster, currently they run at the same speed as ATA150 drives, but iirc they have plans afoot to goto ATA300 and ATA600 speeds in the future.