Admittedly, adding a drive won't give you the speed boost you get from a processor or system memory upgrade. But additional storage space does help Windows' memory caching and gives workgroup members the freedom to focus on their jobs, instead of PC housekeeping.
Not so long ago we paid $1 per megabyte for storage--and not long before that we paid $10 per megabyte. But now drive prices are less than a nickel per megabyte, depending on the size and speed of the drive you select (the larger, the cheaper). In fact, you can get 40GB of storage in Maxtor's new DiamondMax Plus 40 for about $350--that's less than a penny per megabyte. You only have to look at the system configuration Windows 2000 requires to know that you'll need every byte.
Unlike some PC upgrades, you can add a drive without throwing away the old hard drive. Each of your office PCs is probably equipped with a drive controller able to accept one, or maybe two, additional IDE hard drives. If not, you may have to add another drive controller like Promise Technology's Ultra66 card, which should be available for under $60, in order to connect an additional drive.
When the inevitable happens and you have to upgrade to entirely new PCs, you can take these drives with you or add them to your local area network. Drives undergo constant improvement; but if they aren't filled up, they don't obsolete as quickly as other PC components.