Generally, we buy the best server we can afford for a particular application, and only replace it when absolutely necessary. They are basically on their own schedules.
For the workstations, we try to replace about 1/3 each year. We, as I suspect most would, start the rotation with the money makers of the firm, and the officers. They can determine otherwise, of course.
Management gets to decide which departments go a what time, or they get to allow 1/3 of each department to go each year. [I prefer the latter, but there is always politics to consider, so we leave the ultimate decision to the people with the money...]
Should this be an across the board [each department gets to replace 1/3 each year] we give them a certain amount of credits they can use to pick out different types of packages; a desktop with a PDA, a laptop, two desktops, etc.
Basically, we offer to guide their use of technology, but leave the decision to who gets what to each director/manager. With everyone involved, things seem to be generally happier.
Good luck.