RPM is the speed at which the disk spin inside the harddrive. They have nothing to do with its transfer speeds or ability. (though the faster the RPM, the better chance you have of it transferring things faster.)
USB2 should be able to do around 30MB/s. Firewire or USB3 would be faster still but not as common. Any external disks withing the last few (10?) years should support USB2.
The term "RPM" refers to revolutions per minute.
It has a direct correlation to access times, but not necessarily to data transfer rates.
Data transfer rates are determined by the density of the drive platter and the rpm of the drive. It is not a constant, but a variable.
The outer rings of a hard drive will hold more data, so the data transfer rate of a drive will become less as you access the inner/slower portions of a drive.
For an external hard drive, your first limitation will be the connection to the external enclosure..
If you use usb2.0, the hard drive is irrelevant, since it is 1/10 the potential of any hard drive.
If you use usb3.0, or Esata, then you will be closer in balance.
Hard drives will have the capability of transferring on the order of 100-200MB/second, so which drive you select does not really matter.