As mentioned before, if it's a server you're equipping, the best drive is the Seagate Cheetah. But if its drive bays are maxed out, you might turn to network area storage (NAS) devices.
Instead of shutting down your network to work within the system itself, you can simply attach a NAS server to a network adaptor, configure it and have it operate like any other computer on your LAN. It can provide storage for all connected PCs with the added advantage of bypassing--and thus, taking some of the file-saving load off--the main server.
It's a new, fast-growing storage category, with Quantum's Snap Server family the most popular option by far. A Snap Server looks like a small, monitorless PC with one or two drives, and it ranges in price from $499 for 10GB to $1,799 for 40GB of storage.
That's pricey as hard drives go, but stacks up pretty well against the $3,000 to $10,000 you'd pay for the alternative--adding another LAN server. Also, adding or upgrading a general-purpose server usually involves shutting down the network at night or on the weekend so work isn't interrupted and paying a premium for installation.
Basically, a drive upgrade of any kind should take up as little of your time as possible. Spend a little more money upfront, have it done by experts, and that investment should pay for itself many times over in enhanced productivity for you and your employees.