Jim all you had to was type in 'secular christmas cards' and you would have found the right one, there's a Mars Dove and a galaxy one on here..

http://www.coolcards.co.uk/acatalog/Atheist_Secular_Christmas_Cards.html#aCP3

Really you should make your own ;) ;), Rhiannon can make humbugs to give away as presents for not entering into 'the spirit' of the thing, and someone can think of something to 'nice' to write in your cards; note that Carl Sagan is mentioned as a possible contender for the written verse...