Before I get on to the actual question there is one point that I think might be quite significant here: in most cases, with modern drives, the data rate of the USB adaptor and the CPU hit associated with high-speed USB transfers are going to be more significant than the spin rate of the drives. There is no point spending power on spinning a disk faster to achieve a bulk transfer rate of 100MByte/sec when the USB transfer probably tops out somewhere between 25 and 30MByte/sec anyway - the faster spin, and therefore the extra power, is wasted.
The faster a drive spins the more power it will need, especially when initially spinning up form idle, and the standard says that a single port can only provide 2.5W of power (0.5A at 5V). For this reason you are unlikely to find 7200 RPM 2.5" drives in USB drives that do not require their own separate power adaptor.
Many "2.5" drive -> USB" adaptors have two USB plugs to allow drawing power from two sockets to pick up more power than the 2.5W one port can provide. Be careful with these and unpowered hubs though, as an unpowered hub is limited to 2.5W output in total *not( 2.5W per port (powered hubs do provide 2.5W per port, and many hubs can operate in both powered and unpowered modes). If I remember rightly USB3 will increase the range of power profiles available, including new higher-power options, but USB3 is a fair while away from general availability at this time.
The main advantage of spinning the disk faster is that more data passes by the drive heads in a given amount of time, increasing the bulk transfer speed. The same effect can be had by using drives with a greater data-per-track density (i.e. the same amount of data on fewer platters but the same number of tracks per platter) so you might find that there are disks which provide better bulk transfer speeds without increasing the spin rate. The other advantage of a faster spin rate (slightly reduced latency for random access) can not be helped by greater data density (though this latency is small compared to that imposed by the physical head positioning time).
Before plugging any drive into a USB adaptor that does not have its own power, check the power requirements of the drive you intend to use. Many drives, especially those that spin faster or are just older, may want to draw more then 2.5W when spinning up even if they need less during active use once powered on. For instance looking up details for the drive I have sat on my shelf here (a 20Gb drive pulled from a dead laptop) it is rated as needing up to 2.9W when spinning up (see http://sdd.toshiba.com/techdocs/MK2016GAPuserguide.pdf) so would not be suitable for plugging into a single USB port.