Just wondering if you have two different hard drives on a raid how badly it hurts the performance of your computer? I.E. 10,000 74 gb western digital raided with a 7400 rpm 80 gb seagate drive.
I assume you mean RAID 0 (striping). I guess the slowest drive limits the speed of the whole array.
If you had a 5,400 RPM drive paired up with a 7200 RPM drive your RAID array will always operate at the speed or capacity of the weakest or smallest drive. A chain is only as strong as its weakest link. So, obviously, if you're looking to set up a RAID setup, buy two identical drives.