Is there a huge difference with lining and interlining?  I'm getting
ready to order 100% silk draperies from dreamdrapes.com and need a
neutral opinion. The drapes I'll be ordering have "sateen lining" and
"heavy flannel" interlining. I wouldn't know the difference between a
low grade or high grade lining if they hit me in the face so I'm
wondering if these are really better than average linings and if so,
what really is the difference? I'm new to having drapes made so I
would appreciate some good advice!

Here are a couple of sites that may help:
http://www.victorianstation.com/curtains.htm
http://www.denverfabrics.com/pages/static/curtains/sewing-curtains.htm

As I understand it, the sateen lining is to the street with the
interlining adding body between the lining and the draperies.