I would expect the systems to run quadcores, quadro gpu's, and at least 4gb of DDR2 RAM? Are your current computers prebuilts or all custom built?
With Revit, smooth performance would desire quadro cards/firepro cards...or simply workstation cards. They cost alot more then the average desktop user cards or enthusiast ones.
If they have quadro's already, I would stick with them over consumer graphic cards.
Better yet, how many do you need?