Advantage Processors credit card processing.

Advantage Processors are one of a number of companies that provide the means for businesses to receive credit card payments from customers for goods and services.

From what I could see from their site there was the ability to process a number of cards such as Visa, MasterCard, American Express, Diners Club, and Discover which pretty much has it covered.
This may be a solution if you are needing to reach people who possess cards other than Visa (which I have noticed may be the only option provided by some online businesses) which may exclude these potential customers who may be keen to buy, but go elsewhere simply because they don't have an accepted form of payment.

The company also claims to have some of the most competitive rates available and deals with a wide range of transactions from retail establishments and restaurants to mail order or online dating. They also seem to be well equipped to deal with home based businesses.
There is also an online tutorial to help their customers understand the ins and outs of Merchant accounts.
http://www.advantageprocessors.com