|Are you thinking of setting up a RAID 0 array which it appears to be as you mention speed. If this is the case then I would urge you to think again.|
There is no speed advantage to be gained and you put yourself in a vulnerable position if one of the disks should fail. There is no chance of data recovery at all. Regular backups are essential. RAID 0 only has a speed advantage if you a regularly reading and writing huge file and I mean files in the giga byte range.
Speed is not the objective of RAID. Redundancy is and there is no redundancy on a RAID 0 array, quite the opposite in fact.
Take RAID out of the equation and it doesn't matter which drive the OS goes on. The difference in the real world between PATA and SATA is so small as to not be worth worrying about.