At this point, I wouldn't consider a 32-bit OS if you're planning for the computer to remain in service for more than a couple of years. Software is going to start getting scarce soon. There are rumors that Adobe will be 64-bit only by CS6. After Effects CS5 is going to be 64-bit only, and PhotoShop is currently available in both 32- and 64-bit versions.

Not everyone is going to be as quick to move to 64-bit as Adobe, of course, but it would not surprise me if it starts getting difficult to find graphic software for a 32-bit machine.

Beyond that, Waldronate's advice is sound—for 2d work, there is little need for a top-of-the-line workstation. If your cartographers are going to be running some heavy simulations (geology and erosion studies, for instance), then processing power will be important. Otherwise, more modest hardware is fine for this kind of application.

One other thing to take note of is service and technical support from the vendor. You definitely want a responsive help line for the customer unless you plan to offer ongoing support yourself.

Oh, and you'll probably want to address off-site storage for disaster recovery purposes. A couple of 1TB portable hard drives that can be removed to another location on a regular basis is not a bad idea, although those would be more appropriately allocated to the budget for the server than the workstations.