Kolab with approx 20 users and all service on the same server - minimum hardware requirements for acceptable use?

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I can't find a minimum recommended hardware suggestion for my current installation needs.

about 20-30 users with all dependant services running on the same server under low -> moderate use.

what kind of performance should I expect running Debian 7 (Wheezy) with an intel D525, 4gb ddr2 667 / 800 + 2ea 500gb sata in raid 1?

I'm trying to get all my servers / workstations low wattage mini ITX in an effort to keep the heat / noise and power consumption down.