There are some good recommendations in this thread:
AdViCe 101-for new brand!
I recommend looking for a provider that uses the authorizenet gateway since it is supported by most shopping carts.
Try to find ones that will offer low monthly fees and low percentage rates.
My personal recommendation is
e-onlinedata.
I've been using them for over a year and I have no complaints. I also have a merchant account through linkpointcentral/cardservice international and I prefer the e-onlinedata/authorizenet way.