K... here's the deal :-)
Hard drives have two main components that are important as far as speed... one is access times and the other is transfer rate. Rotational speed affects access times and the interface (ie: ATA100, or ATA133) affects max transfer rate. This is all general, btw, lol, so take it as such. External hard drives at 7200 rpm are faster with access times because those drives usually have read/write times of like 8 milliseconds whereas 5400rpm drives are around 13ms and 4200rpm are like 19. Considering the number of times the hard drive has to seek, those times really REALLY add up. So, if you have an external 7200rpm drive on firewire or USB2, you get the benefit of faster seek times. This is good for things like searching, changing files, basically doing things that involve seeking :-) Now, modern drives are generally on an ATA100 connectionl, which means they have a max transfer rate (MAX) of 100 megabytes/s. Firewire maxes at 50 megabytes/s and USB2 maxes at 60. Hard drives average at transfer rates of 35MB/s just depending on what is being transfered and where on the drive it is being transfered from. So, USUALLY, firewire is enough and you will only notice an increase in performance compared to the current 5400rpm internal laptop hard drives. Once 7200rpm drives come out, there will be no performance reason to have external drives because performance would theoretically be less (cuz of the bandwidth limitations on firewire and USB2). Hope that helps!