The RPM of a hard disk relates to how fast (in revolutions per minute) the disks that contain information spin. Now, because a separate mechanical interface (a reader/writer head) is used to read and write information to this disk, the faster the disk spins, the faster the read/write head is able to get information on and off the disk. (theoretically, of course - there are other limitation factors, but let's keep it simple) A 4200RPM disk is significantly slower than that of a 5400RPM disk. Therefore, in situations where data needs to be accesses as soon as possible (like when you're fretting to find the next song to play in the last seconds of a currently-playing song), a 5400RPM disk is favored because it can get to the information much faster. Ideally, you should be using a 7200RPM disk (or an SSD, if you've the money), but 5400 will do fine. 4200 disks, though, are noticeably more sluggish than their faster counterparts, and I think you'll find that, if you use one, you may run into trouble in certain situations.