Number of Users as a Requirement for Your Lion Server Hardware
The effect of an increasing number of connected clients on OS X Lion Server performance isn’t linear. You may not notice slower service as the number of connected clients increases until you get to a tipping point, when performance suddenly slows to a crawl.
With up to ten or so users, the lower-end Macs can handle multiple tasks at once. If you add more users later, you can always add more lower-end Mac servers for other tasks. Current iMac models have faster processors and architecture than the minis, can hold more RAM, and can handle more clients.
The top-of-the-line Mac Pro can potentially handle hundreds of clients, again depending on what services you run. With the Mac Pro or an older Xserve, adding more higher-end storage or large amounts of RAM can help enlarge the client load that the server can handle. Network capacity (number of Ethernet cards and their speed) is also important in serving large numbers of clients.