I'd suggest you might want to keep three things in mind as you frame pieces for a show: they need to look like they are all of a high quality (not necessarily BE expensive, they just need to look like it!), they need to go well together as a group, they should be simple and classic enough in style that any client could envision them in their home.
I would get the very best frames you can afford and avoid the metal ones completely. Presentation is everything. I really mean that. A great frame can make a so-so painting look terrific and a bad frame can make a terrific painting look really crummy.
Check out
www.jfmenterprises.net for some good quality standard size frames for paintings at great prices. You need to have a tax I.D./wholesale number to get access to their price list but as a professional you should have one of those already anyway.