Let me repeat: a RAID array appears to Windows as a single drive. That's
independent of how many physical drives make up the array. You can set up
the whole capacity as the C drive if you like. That's true whether the array
is a 2 disk RAID 0 array, or 6 disks in RAID 5.
(Incidentally: if you're creating a RAID 0 array from two drives, the
capacity of the array will be twice that of the smaller of the two drives.
It's best that the drives be the same size, so that none of the capacity
will be unused.)
You create and format a partition during the Vista installation. If you want
multiple partitions, you size the installation partition accordingly. I
believe that I created and formatted the other partitions later, using the
disk management utility. (That's under the "administrative tools" control
panel, computer management, disk management.) This is exactly like you'd do
with a single drive.