>Also, which is faster a 120GB 5400RPM or a 60GB 7200RPM drive? The 120GB has higher density while the 60GB has faster spindle speed.
The main thing spinning the HD faster does is to decrease the time it takes to spin to the right spot on the initial seek/read. On commercial servers, where time is money, this can be a very big deal.
The max data rate feasible is governed by the bandwidth of the data separator, which is an expensive part of a HD. On consumer oriented drives they are probably the same on 7200/5400 drives, so the data rate used would be the same, resulting in lower density on the platter for the 7200. However spinning a drive faster makes the signal the head picks up stronger, improving signal to noise ratio, which should allow slightly lower, and therefore slightly denser, magnetic domains on the HD, without losing signal qualtity. But you would need a data separator with a higher bandwidth. On price-is-no-object SCSI drives, they use the best data separator they can obtain. That wouldn't make much sense on a $100 HD.