I'm networking a Mac OS X to a NT 4.0 server, and I need to know if you can share existing folders/files with the Mac OS X or only the folders that you can create through AppleTalk?

on the nt machine with networking services for macintosh installed, you can turn shared folders into MacVolumes which would then be accessible by both MAC and PC's.

go to server manager, select the appropriate server, go to MacFile on menubar, click on create volume, enter the name you want the volume to be called, enter the path to the shared folder and there are some options at bottom you might want to select/deselect. You may get a message about the volume being greater than 2 GB and may be inaccessible but that is for older MAC O/S's. Once the volumes are created, go the MAC machine and login with microsoft authentication, and you will get a list of volumes available so you can select the one you want. NTFS permissions apply so if the logged in person does not have any rights to that folder, it will be grayed out.