Certainly capacity is an important issue, but there's more to desktop storage than that. The other side of the platter is performance.
Our sister publication Computer Shopper has put together an extremely useful guide to buying disk storage. If you haven't thought about storage issues for a while, it would be worth your time to review this information.
To get an idea of the best-performing drives, look at their average sustained transfer rate and average access time. All other performance specifications, in the words of Computer Shopper, "matter only because they affect these two key specifications."
The best way to test drives is to put them in PCs and try them out with the applications you use most, but that's really not worth taking the time to do; any recently released drive will be adequate.
You don't need the best-performing disks available, but you do want to be on the high end of the curve if you plan to keep the computers they run gainfully employed for the typical three- to five-year life of most business machines. My computer at work would have been an average-to-good performer three years ago; today I think it's a dog.
A more important issue for most businesses is value. That means looking not only at the price of the drive, but also at the price per gigabyte of storage, which can run from less than $4 for very large drives to nearly twice that. If you haven't bought disk drives in a while, that still may sound pretty good; disk prices, like those of virtually all components, are dropping all the time.
Also think about manageability as you prepare to roll out new PCs or drives. I suggest installing a desktop management suite such as Intel LANDesk. Its centralized administration, inventory, software distribution, and remote control tools can be worth as much as another administrator in improving your support department's ability to aid its users.