A bit more information is needed to really make a good recommendation for this. You really have two options from Microsoft - Server 2003 and Small Business Server 2003.
The minimal configuration I supply to small businesses is 3.0Ghz P4 with 1Gig RAM, a pair of mirrored 120Gig Drives, a Sony Tape Drive for backup, DVD burner, Intel Motherboard with Dual 10/100/1000 NICs.
If the budget allows multiple servers than split them up. Otherwise, a single server may well fill the bill.
Questions -
Will there be remote access to the data (not for remote managment, but needing Terminal Services for users)?
Is your mail hosted locally or with an ISP? Do you want or need an Exchange server?
Do you need a shared contacts list?
Would an internal web server make sense to the organization?
The workstations will have the software for the applications, not the server. Each workstation must have a copy of what ever office suite you are using. Can they or will they consider OpenOffice.org/StarOffice, or do you have to go with Microsoft Office? Remember, unless you are in a non-profit environment, this gets expensive.
If you plan to be in a Microsoft environment (doesn't sound like Linux is in consideration - shame) then put XP Pro on the workstations.
Size of the RAID? How much data do you have and what growth do you expect? Double that number for your initial storage requirement. Most users will not take a document, put it on the server and then point everyone to that document, they will email a copy to everyone, and each user will keep the original email as well as a copy of the file.
As for the type of RAID - again, what kind of budget do you have? Small companies can get away with simple mirroring. This protects against a catastrophic failure of one drive. A stripped array (RAID 5) is much more robust, and if you have hot swappable drives then all the easier to maintain, but it does up your hardware costs considerably.
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