Okay...there is a rule of ettiqute that says do not send a religious card to someone who is a different religion than you. Reason being you can't understand the signifagance really But, I think it would be nice to get a card from someone that mentioned my specific beliefs.
i usually give cards that are pretty generic. ones that have humor attached to them cuz i am not a big fan of the serious, all script, poetic xmas/holiday cards. but that's just me...
I try to keep mine pretty generic or pretty generic (ie. God Bless), mostly because much of the time I don't know the religion of people I'm sending them to (work associates, etc).
Really, though... I'm Catholic, and I certainly wouldn't be offended if someone sent me a "happy hannukah" card, though... these things happen.