RPMs? Depends on your perspective, generally speaking faster is better so 7200 would be better than 5400, just not quite as critical if you're just using it for backup purposes. Faster is never better when it comes to heat disipation so keep in mind, no matter what you select make sure that the enclosure has at least a large mesh area to help with heat disipation and ideally one with a cooling fan as part of the enclosure would be good, or take a look at some of these "Docks" and purchase the hard drive seperately. And definitely stay away from those small form factor drives that are literally sealed in a plastic tomb with no ventilation at all. I believe the link to the 500 gig above is an example of one of those. I had a 160 just like that and it was great, nice small compact could carry it around it my shirt pocket. Lasted about 4 months and I'm convinced it was heat that took it down.