Your machine will be as fast as the slowest drive.. yes like they said above, you should have ideal drives if possible.
The drives can be different sizes, If you have a 100gb drive and a 200gb drive,, when you stripe them, it will use the 100gb drive and 100gigs on the 200gig drive, wasting 100gigs.
Also, you have access times to think about. If one drive can access data in 7ms, and the other one takes 8ms, you will have a bottleneck issue and the system could seem sluggish.
As far as the drive cache, i am not surehow that works in raid. not sure if the cache works like a stripe or not.
And definitly make sure that the drives rotate at the same speed..
.............Just to reiterate what they said above
Raid0 = good performance, no redundancy.. Must have at least 2 drives
Raid1=crap performance, Good redundancy..Must have at least 2 drives
Raid5=good performance, Great redundancy..Must have at least 3 drives, and you loose some storage space
Help this was educational lol