For external drives, the bottleneck is really more about your interface rather than your RPM speed.
A SATA drive connected via USB2 or FW400 is going to be much, much faster if connected via FW800, or eSATA.
What kind of system do you have? If you have either eSATA or FW800 - a lot of the 7200 RPM SATA drives freaking fly, even if connected externally.