If the issue is purely security, I'd look at Linux - you can get free to low cost Linux that will run quite well on existing hardware (if the hardware is otherwise acceptable), which reduces the cost of the move as well. If you have budget, I'd look at something like SLED (Novell's offering) because it's designed to for networks and manageability, but you could get a free version as well. I'm not sure that a free version will really cost you less in the long run than a well supported paid for distribution.
Some things to look at:
Whether the choice is Linux or Mac what kind of interoperablity do you need. Basic MS office compatibility can be had on either MAc or Linux (Open Office is quite good that way), but if you MUST have compatibility with complex Office documents, you need to look at your options a bit more closely. I am fairly certain that both have options for this, but there are differences in cost and complexity.
What other things do your staff need to do? Linux is strong in some areas and weak in others - same for the MAC. This is especially an issue if your staff are doing something unusual or have special needs, For examples, if you have staff with vision issues, you need to check that out. Or if you staff is monitoring internet radio stations as part of their jobs, you need to make sure that you have the right codecs for that (This one is morel likely to be an issue in Linux than Mac at this point.) I wouldn't worry about the possibility of special needs in the future, to be honest, because there is so much development going on.
Another aspect of this is what organizational systems are you using? Which accounting system are you using? What about program management databases? etc.
Something to think about is going to Linux or Mac on the server, and staying on Windows on the desktop. Not quite the same as a full move, but it may be more practical.
Kayza Kleinman
Director
Nonprofit Helpdesk
Jewish Community Council of Greater Coney Island
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