If your PC is non-Intel it will not work. Twincat only works on a Intel based computer. If your PC is Intel processor it does not matter what ethernet card you use as long as it works in windows it will work with twincat.
The way I understand it ethercat has more to do with how the hardware nodes recieve the packets and the protocol and not the ethernet card in the computer. I could be wrong though I will look into it more.
Still all NICs are not working properly, that´s my experience. I took part in project were they had to replace the NIC to another type that was recommended by Beckhoff, I do not remember for what reason but for sure there was a problem with that card, and that project wasn´t first time.
I think that´s about two years ago, maybe they solved "problem" with uncompatible NICs now.
In theory,yes. However, somewhere in the depths of Beckhoff documentation there is a mentioning that the proper real-time communications are guaranteed only with certain types of network cards. It could be any standard brand, what matters is the chipset used.
Intel's is for sure; I think one of the Realtek chipsets is also supported. When you open TwinCAT System Manager and try to configure an EtherCAT system, you will have to choose a network card for communications. A list of all the network cards available on the PC will be presented and they will be split into "Compatible" and "Incompatible". You can enable EtherCAT on a compatible card and get true real-time comms; the system will allow you to use an "incompatible" card, but real-time capability will be limited.
It is always a good idea to run ECAT off a separate network card, network cards are cheap these days (you don't need a Gb card, ECAT at this time is 10/100 Mb only). Just look for a card with Intel chipset so you don't have to worry in the future.