Another question: How many drives do you need in RAID5 before you get back to the performance level of a single drive? I have seem some reports here that 3-disk RAID5 is unexpectedly slow.
sminlal is correct, mostly. you'll see read speeds more/less equivalent to reading from a RAID 0 of 1 less number of drives, after all RAID 5 is essentially just a RAID 0 with an extra drive for the fault tolerance of parity. writing is much more difficult because parity must be calculated from an intensive XOR formula, so writing is usually limited to the speed of the processor calculating parity. a software RAID5 (that relies on the CPU) will be slower than a hardware RAID5 (controller card that has processor dedicated to handling calculations itself).
i have hardware accelerated RAID5, 4*1TB drives (3TB logical drive). i'm pushing 200MB/s both read and write.