Thank you letter after an interview...

I have read a couple of good advice in sending a "thank you" letter after an interview- they're very nice advice... thank you guys! I just got my interview 1 hour ago and I'm just done making a thank you letter for my interviewer. I definitely will send this but my question is that... shall I write it in a little thank you card? or write it on the piece of blank white paper? which one better? Thank you!
Either is OK ... but I would do it with a professional looking "Thank you" card.
I always use a blank "thank you" card. I've had good results with that.