There isn't a "standard" RPM as such. Most manufacturers will make disks that spin at 5400, 7200, 10000 or 15000 RPM. The faster the disk spins (higher RPM) the lower the latency. Latency is the time it takes the disk to bring a requested sector under the read hear, so lower latency is a good thing for a DB.
Also, depending on what you are doing, slower disks may be absolutely fine. For example, if you have archive data that is rarely accessed, it probably isn't worth shelling out on the fastest disks imaginable