The real bang for the buck is to remove the sheetrock in the garage and use open cell foam on the underside of the room above.
This is the most effective way to warm your tootsies upstairs.
For more information:
Read Harold Remlinger's Q&A "How should we heat and cool our new garage addition?"
Also, read Jesse Terzi's Q&A "How can we prevent car fumes from polluting our house through the attached garage?"