Really? What a waste of time!
If you want support Linux, get the software and use it. Buy a t-shirt to show everyone, if you want, the merchandise profits may help thre foundation, the t-shirt will be seen by all and sundry more often.
Is this pointless card really going to stir up your average person to dump windows and try something new? No!
Is the average shop assistent going to suddenly start a conversation about the latest release of Gnome/KDE? 1 in 50,000, maybe?
Linux people get this: 98% of people who use PC's don't give a toss what's on it, so long as the browser ( preferably IE on Windows ) gets to Facebook, Youtube, the news site of choice and Hotmail/GMail and the PC can run iTunes to get ripped off music on their iPod, they simply don't care! Let me repeat that for the Linux hard of hearing, THEY DO NOT CARE! OK?
( I use Linux day in day out as Unix SA/Oracle DBA and I use an Apple Mac at home, so don't start flaming me as a fanboir baiter! )