This question talks about different payment processors and what they cost, but I'm looking for the answer to what do I need to do if I want to accept credit card payments?

Assume I need to store credit card numbers for customers, so that the obvious solution of relying on the credit card processor to do the heavy lifting is not available.

PCI Data Security, which is apparently the standard for storing credit card info, has a bunch of general requirements, but how does one implement them?

And what about the vendors, like Visa, who have their own best practices?

Do I need to have keyfob access to the machine? What about physically protecting it from hackers in the building? Or even what if someone got their hands on the backup files with the sql server data files on it?

What about backups? Are there other physical copies of that data around?

Tip: If you get a merchant account, you should negotiate that they charge you "interchange-plus" instead of tiered pricing. With tiered pricing, they will charge you different rates based on what type of Visa/MC is used -- ie. they charge you more for cards with big rewards attached to them. Interchange plus billing means you only pay the processor what Visa/MC charges them, plus a flat fee. (Amex and Discover charge their own rates directly to merchants, so this doesn't apply to those cards. You'll find Amex rates to be in the 3% range and Discover could be as low as 1%. Visa/MC is in the 2% range). This service is supposed to do the negotiation for you (I haven't used it, this is not an ad, and I'm not affiliated with the website, but this service is greatly needed.)

This blog post gives a complete rundown of handling credit cards (specifically for the UK).