The space you would gain would be so minor it wouldn't be worth it.
Afaik though there isn't a program for you to have them on the SD card unless you get a seperate 3rd party contact program that has that feature. I know there is a program or 2 out there that will backup your contacts though and you can have it save the file to your sd card. Can't remember the name though as i dont' have it installed and i'm at work.