I think that there is a square block on the back of the MAC (right side)that you "knock out". There is a card that you install on the motherboard, I belive you need an LC Slot Ethernet Card. I would suggest going with an Asante NIC. If you go the to Asante website, I think they will tell you what your options are for a 575 (I would guess the NIC costs around $75). I would NOT suggest buying ONLINE as you will need tech support if you are asking these questions. I would also suggest using System 7.6.1 too while your at it. If you only need TCP/IP for internet access, then your NT server will do fine running DHCP, if you want file & print sharing, I would check into this further. I've heard the NT folks say adding Mac Services is a surefire way to wack your already unstable NT into minute by minute crashing NT server. I can tell you that Netware 4.x and up works well with Mac Services from my experience (I've worked in K-12 schools for over 8 years as a network admin).
Hope this gets you down the right rode....