Elroy thinks that all of the posts to this point are missing the most important issue. If you're going to purchase a c-clamp, make sure they are Forged as a cast or ductile iron c-clamp typically won't have near the strength as a good drop forged model.
You'll also want to look at the screw. A forged clamp will have an acme thread verse a machine thread. Copper plated screws along with notched pressure pads are also a nice feature.
Other options to consider in addition to a standard C-Clamp are the adjustable frame clamps typically referred to a Bessy clamps as Bessy was one of the original manufactures.
A nice multiple selection of clamps always comes in handy.
Let Elroy rephrase that:
A nice multiple selection of
forged steel clamps always comes in handy.
Wilton offers some very nice pieces. Look at the Bessy brand as well and you'll find a nice selection to choose from.
http://www.besseytools.com/price_vs_cost.htm