USB HD would be slower even if the external HD was a normal 7200 RPM HD. Firewire would help and be a bit faster than your normal USB connection. But still I think your internal 5200rmp HD would be faster than any external HD you buy. (unless you have external eSATA connection)
I think the best solution for you (if you are willing to spend a bit) is to get a new 7200rpm HD for your laptop. These days Laptop HDs do come in a 7200rpm flavor.
Take out your old 5200rpm HD and use this slower HD as your USB external HD to put pics MP3s etc. Who cares how fast your external HD is because your limited by the USB, and a 5200rpm laptop HD would be a perfect external HD its small, quiet and cool.