Can I use Snow Leopard Server with Windows clients?

Yes, you can.

In general, it'll be easier to run Windows Server here, unless you're planning on a migration to Mac OS X or to other Unix or Linux systems, as you'll be learning and managing and maintaining two very different platforms. ( (Or more than two, if you see Windows XP management as being different from Windows 7.) Windows clients are integrated with Windows Server boxes, and you can run Exchange Server and Sharepoint and similar here.

As for Mac OS X Server and its Mail server? Sure. Works fine.

Calendars via iCal? I'd expect that works, but haven't tried it. Certainly the web-based or likely via the Mozilla Sunbird or Lightning packages.

As you add more requirements than calendar and mail and DNS services, or add web and baseline file sharing and such, then Windows Server increasingly becomes your best choice. Authentication, for instance, is easier with Windows Server running Active Directory or with a so-called magic-triangle configuration of Mac OS X Server and AD.

I'd suggest reducing the amount of complexity here, not adding to it, too. Fewer Windows versions, fewer Office versions, etc. Maybe move to cloud hosting via Google Docs or any of the providers, depending on the sensitivity of the information, too. Or maybe toward thin clients and fewer servers. A dozen different configurations and a network and multiple servers can be an investment in time, and can be heading toward needing an IT staff and particular as this scales.

No problem, a new server is coming next month, but it will certainly be the best thing you've done for yourself and your users.

Can Mac OS X Server do this? Sure.

You're going to be learning a whole lot about running a server, regardless.

You really need to sort out what you have here and where you want to be, and how to upgrade or replace yor migrate your environment, and before you add the substantial increase in complexity of serving Windows from Mac. If you can't get off of Office circa 2003 or similarly old software, what are you going to do when you find, say, an incompatibility, or a need to migrate mail clients? Who are you going to call for help?

I'd guess that Small Business Server (SBS) is likely your best target here.

AFAIK, Exchange Server (which I'm guessing is what you're using) supports larger mailboxes in newer releases.

Look at the sustaining costs for what you're considering purchasing, too. How much it'll cost to configure, deploy and maintain the box and the network. Up-front costs are an obvious consideration, but maintaining many boxes and many versions itself introduces costs.

The arrival of Lion Server next month does not change my opinion. (And I'm not usually suggesting Windows and Windows Server boxes, either.) (And with the arrival of Lion and Lion Server, I'll follow my usual approach and not look to upgrade to the first release of that (or any other) new platform, if there are business-critical functions and services involved. You need time to debug and test the deployment.)