If you'd like a photographers advice :-) then the absolute best online digital printing site right now is
www.Mpix.com.
If you want larger than 20x30 you will have to find a poster or large print specialist, I think I know of a couple but I'll have to double check.
If you're willing to wait a little longer and do it via mail, Dale Laboratories (
www.dalelabs.com) is fantastic and they do thing such as professional Gatorboard mounting and the like, both digital and film. Those are the two places I use all the time, for both my work and when printing hi-res NASA photos.