If it's any help, I'm going through the process of finding one right now. It's difficult to find all the info you need in one place. Merchant account necessities obviously vary from business to business, but here are a couple resources that I think were helpful...

Merchant Account Reviews (it was tough finding a company that actually reviews merchant accounts without being biased towards whoever they're partnering with. the above site seems pretty good though.)

Merchant Council (just some good general info here.)

Good luck!