with the mini's tiny form factor - i'd consider the possibility of overheating if you put too fast of a drive in. may not matter so much since people have put 7200rpm drives in powerbooks but just something to consider before you dive in.
Be that as it may, it is the lowest end model of Mac. So componants are as cheap as cheap can be. I agree that I'd be careful how fast a hard drive I put in, there's no telling how much heat a 7200RPM drive will generate and how the mini can cope. I imagine there's some info out there on xlr8yourmac or some such site..