Found some "how to write business letter" sites that left me with some questions. One site claimed you should end letters with "yours faithfully" if you don't know the person and "yours sincerely" if you do know the person. I've never heard of this--is it common business practice in the US?

Also, some examples had date after addresses and other examples had date after sender address but before delievery address. Are both formats acceptable?
There's something similar in English - almost identical I think..
Yes, you should use Yours sincerely if you have addressed the letter to a named person, for example Dear Mr Smith. It does not matter if you personally know that person or not, it is a matter of whether you know their name.

If you are addressing to just a Dear Sir, or Dear Madam, without an acutal name, you should use Yours faithfully.

This is common practice.

Also, some examples had date after addresses and other examples had date after sender address but before delievery address. Are both formats acceptable?

Put the date beneath any addresses (you do not always include a typed sender address, for example you are probably using headed paper for business correspondance).