Another note here: you can put TV cards anywhere, in the core and/or in media directors. Sometimes, putting a TV card in a media director makes sense, as most of the time, there already is an antenna outlet near your TV.
No matter where your TV cards are, any media director is able to utilize any TV card in your whole setup.
Regarding 100mbit vs. 1gbit networks. Make sure you have a QUALITY switch. A good quality 3Com, Cisco or HP switch 100mbit, will be better than a cheap run of the mill gigabit switch.