There is no no need for an "apple talk NIC". A standard Ethernet controller should work fine. You will need to load the appletalk protocol and Services for Macintosh (SFM)on the NT server.
The first compatabilty issue is does your network support appletalk? Most likely it does (since you already have the Mac's) Aplletalk is not one of the fastest protocols... See the following articles in microsofts tech net: Q124655,Q252419 (for windows2k) and Q101747 (how to set up authenication from apple clients)
NT and MACs mix, but not that well. If you have an all MAC shop- I would look at Apple's OSX server. With the future OS releases for the desktops falling in line with the OSX model with a unix kernel- it may be a better solution to go in the long run for an all Mac shop.
Good Luck! SFO