I've heard Costco has really great rates. You'll probably want to do some comparison shopping. Also, be sure the gateway they use is compatible with 3dcart.
Some fees to pay attention to are the monthly access fee, the statement fee, the processing % fee and the per transaction fee.
Generally, you'll pay a monthly access fee around $15 and a statement fee around $10 (estimates). For each transaction, you'll pay a percentage and a per transaction fee. PayPal's basic rates, for example, are 2.9% of the transaction total plus $.30 per transaction.
I've used
www.eonlinedata.com for a couple of years and have been happy with them. They offered the best rates I could find, but definitely do check with your bank too as they might have incentive programs for existing customers.