Good advice by Chris.
If you want to take this advice a step further, you can ask each company to provide you with the names of some of their recent clients, whom you could contact directly and find out first hand how they felt about the quality of the work done, the level of support received, and so on.
I have to tell you though -- most people don't even bother with this step of calling a company's references. But if you do go ahead with it, the feedback obtained from clients might prove to be quite informative in helping you decide which firm to choose.
And remember also to check out
TechFinder, as Chris suggested.
Yann