A capture card will only capture a video signal.
You wish to connect your TV Antenna I guess so you will need a TV Tuner Card. This will take in the RF Signal from the Antenna Cable (many different TV channels carried on the same cable) and then will enable you to tune to channels and be able to stream these to different media directors if it is installed in the core. If you want to view more than one Station at a time you will need to have the required number of tuners. Some TV Cards have multiple tuners or you can use multiple cards. You can check the wiki:http://wiki.linuxmce.org/index.php/Category:TV_Cards
Or the MythTV site to give you some clues. Better to go with something that is known to work rather than try and hack something that may appear to be a bargain.
Don't forget the cards will cover DVB-T (Regular Digital TV), DVB-S (Sat) and DVB-C (Cable TV). If you get HD channels then you will need a HD Card which will allow you to view SD Channels too. If you want to view more than one type then you will need additional cards.
Alternatively you can use a Set Top Box under the control of LinuxMCE and then use a video capture device (which will capture Composite, S-Video or maybe even HD Sources) but you won't get as much functionality with LinuxMCE. For example being able to view the program guide and book events and series to record.