On a modern server system (mid-range HP server with HP Smart Array 640x
64-bit, 133MHz PCI-X controller and 15.000rpm drives) will the read
performance still increase with the number of drives used? Is there a
"sweet spot" of the number of drives to be used? Or do other factors
come in play on modern hardware (bus speeds, number of driver per SCSI
channel, cache on the controller)?

Also, will the write performance decrease more with the number of drives
or is the RAID5 parity overhead fixed?