jwolthuis - 2:27 pm on Nov 3, 2011 (gmt 0)
Regarding 2Checkout, please read their Supplier Agreement, located under "Documentation" on their Support page.
They are a "broker"; not a merchant account provider (MAP), nor a gateway to one. They "buy" the items from you, then immediately resell them to whoever visited your site.
You upload your "inventory" to their system, and any orders are technically sold to 2Checkout, with a ship-to address of the shopper. It's much more involved than simply establishing a merchant account with a bank, and tacking-on AmEx and Discover.
Based on my experience in the US, I'd go with PayPal Website Payments Pro. They accept payments in the top-18 world currencies, and the funds post to your PayPal account immediately.
If you also sell on eBay, you can use the same PayPal account to accept credit cards on eBay, using their Payflow Pro API (PayPal bought Payflow from Verisign 6 years ago).