Mac OS X has a slew of file sharing and networking options you could use. Since you'll be working with both Macs and Windows-based PCs, then you'll have to settle for the least common denominator, and that would be choosing Windows file sharing (technically called Samba or SMB).
I've configured Samba servers on different OSs before, in particular Linux, BSD, and Mac OS X. Samba can be configured to allow a maximum number of connections to a share (the term used referring to the device/folder being shared), or even certain users only. One graphical editor you can use to modify the configuration file is SWAT (Samba Web Administration Tool), a web-based interface that can be installed on the Mac.
As fond I am of Panther, it has its weaknesses with SMB file sharing, but making it act as a server shouldn't be a problem.
One nuance you should know about with regards to Windows file sharing is the concept of share access levels. Windows has traditionally had two levels: share-level access and user-level access. Share-level access (SLA) is prevalent on Windows 9x/ME-based systems, and user-level access (ULA) is the default in Windows NT/2000/XP. The difference is simply that SLA simply asks for a password to gain access to a share (should one be provided), whereas ULA requires both a username/password combination (again, should one be provided).
Mac OS X by default uses ULA, which means you would have to add a particular user in Mac OS X (through which you can configure to be given Windows file sharing rights) and give it a password. In turn, Windows clients should have access to this username/password account if they want to access the shares on the Mac. There are ways to make Samba use share-level access on the Mac, but it isn't as straightforward as I expect it to be (or at least work reliably).
To help things out, I'd also recommend downloading an application called "Sharepoints", a free system preference pane that allows easy configuration of files/printers to be shared.
If you really want to ease administration, I'd recommend a product called
DAVE from Thursby Software. DAVE allows your Mac to easily be configured to act as a client/server for Windows file sharing. For me, it at least solves the problems introduced by Panther. You can download a demo to see how it performs for evaluation purposes.
There are other means and ways, of course, to have your Mac do file sharing. You could always settle for things like FTP, or if you're feeling adventurous, set up your own iDisk-like service internally.
Oh, and about your other Mac clients being left out, they needn't be. You could have the Mac OS X system act as AppleTalk/Apple File Server as well, which to me is a far simpler way of file sharing among Macs. Windows has support for that, but you'll need at least Windows NT or Windows 2000.