Hi Joche,
I guess the choice of card depend on the size of your wallet etc.
I have a Twinhan Ter-CI which can take the Free-To-Air channels as well as Boxer channels, see
http://www.twinhan.com/product_terrestrial_3030.asp. It has worked really nicely and costs about 1200 SEK. Not that many cards support encrypted channels so consider what channels you'd like to watch.
Remember, you need a reasonably good signal from the antenna so this may be an area to look into as well if you have limited budget.
You should also check the list of supported TV-cards here:
http://wiki.team-mediaportal.com/Supported_TVCard_DVBt
Hope this helps!
/Peter