The hard drive is an important part of your computer. They are responsible for the conversion of bits into magnetic pulses and back for the storage and retrieval of the information. If anything, a higher RPM means you'll have a hotter drive so you might want to consider bolstering your case cooling.
Arial density is what matters when shopping for a hard drive.
Note that every track has a different length as the platter is circular, so not all the tracks will have the same density. With that all said and done, the history of the hard drive is interesting. Magnetic tape is still used today, though it better suits long-term storage. In '73, the IBM 3340 is heralded as being the predecessor of the modern day hard drive. Physical hard drive formatting has to be done in order for logical formatting to occur.