Some drive manufacturers will crow about the speed of their drive mechanisms. While it is true that a 7,200rpm drive is inherently faster than a 5,400rpm drive, you will only see an improvement in throughput if the drive is connected internally to the PC on the motherboard, or with an eSATA cable. USB 2.0 and FireWire 400/800 interfaces aren't fast enough to handle the raw throughput of a 5,400rpm drive, let alone a SSD, 7,200rpm, or 10,000rpm drive. I'd only worry about the rotational speed of the drive if you're eventually going to remove that drive and pop it in your laptop or desktop (like in a backup/recovery/upgrade kit).
Just wait until the USB 3.0 hype starts. USB 3.0 will certainly bring considerably faster throughput than USB 2.0, but don't expect any mechanical drive to give sustained transfer rates anywhere close to the theoretical 500MByte/sec that it's capable of.