I'm researching tarps and I'd just like to get some advice or suggestions on the subject. The kind/type of ground cloth is the main thing I'm concerned about. Should I have a bivy with a ground cloth? I really don't have any experience with tarps. I'll def be in areas with rain and maybe snow. Weight and compactness means a lot to me.

This is probably the best tarp I've found:

http://www.gossamergear.com/cgi-bin/gossamergear/SilTwinn.html