I am currently creating 175 wedding invitations for my son and his fiancee. It is a 6x6 pocketfold with a pocket on both sides for inserts. I ordered the paper from Anchorpaper.com and I would highly recommend them. Their prices are the lowest I have found anywhere. You can buy full sheets of specialty (metallic finish) paper from them. Most of these sheets are 26" x 40", so you can have them cut down to the size you need for the pocketfold. I am using cover weight Aspire Petallics paper for the pocketfold, which I had cut to 6" x 20" for the pocketfold. I'm using Stardream paper for the inserts (the invitation, RSVP card, etc.). I got a lot of information from posts in this forum from other people who were making invitations too. Here's a couple links to threads I've been following:
Forums at Splitcoaststampers
Forums at Splitcoaststampers
I'd suggest searching this forum and the gallery to find examples of invitations others have done (and to avoid the pitfalls they discovered along the way!).
Kvillegirl was correct about the square envelope costing more. Until May 14, it's an addl. 13 cents, after that I think it's going up to 17 cents. My future daughter-in-law wanted the square shape -- and you know the bride always gets what she wants! :p