IOP values for harddisks

Device	Type	IOPS	Interface	Notes
5,400 rpm SATA drives	HDD	50-80 IOPS	SATA 3 Gb/s
7,200 rpm SATA drives	HDD	75-100 IOPS	SATA 3 Gb/s
10,000 rpm SATA drives	HDD	125-150 IOPS	SATA 3 Gb/s
10,000 rpm SAS drives	HDD	140 IOPS	SAS	SAS is full duplex
15,000 rpm SAS drives	HDD	175-210 IOPS	SAS	SAS is full duplex

In a mirrored configuration:

Disk IOPS = Read IOPS + (2 * Write IOPS)

In a parity (RAID5) configuration:

Disk IOPS = Read IOPS + (4 * Write IOPS)

Example calculations

Now let's look at an example. If you estimate that you need to support 40 Read IOPS (40 reads/sec) and 80 Write IOPS (80 writes/sec).

If you want to use a mirrored configuration of drives:

Disk IOPS = Read IOPS + (2 * Write IOPS) = 40r/s + (2 * 80w/s) = 200 Disk IOPSUsing 7200 rpm drives, you need: 200 / 50 = 4 disk drives
Using 10k rpm drives, you need: 200 / 130 = 2 disk drives (always round up)

If you want to use a parity (RAID5) configuration of drives:

Disk IOPS = Read IOPS + (4 * Write IOPS) = 40r/s + (4 * 80w/s) = 360 Disk IOPSUsing 7200 rpm drives, you need: 360 / 50 = 8 disk drives (always round up)
Using 10k rpm drives, you need: 360 / 130 = 3 disk drives

An excellent article on how how RAID levels effect IOPS 
http://skabelka.com/node/125
http://sudrsn.wordpress.com/2010/12/25/iops-raid-penalty-and-workload-characterization/