As far as gateway goes... we've used them all at one point or another for our own business.
Currently, I'm a fan of eProcessingNetwork for a lot of reasons. Built-in fraud velocity filter & recurring billing plus their Authorize.Net emulation mode are a few of the big reasons.
There are lots of good gateway options though. Authorize.Net is well known and universally compatible with virtually every 3rd party e-commerce app ever made.
iTransact, Plug-n-Pay, CyberSource, Verisign, Paradata... each of them has features & benefits that are useful and price points that match the needs of various types of merchants.
Keep in mind that with all of the above options you still need a compatible Internet merchant account to go with it. You'll also want to make sure that the gateway you select is compatible with any current or future planned e-commerce applications that you'll be using. (i.e. shopping cart, billing software, etc.)
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