Hello,
I have a month to gather all the documents and things. I asked some friends to write letters saying they know we have a valid marriage, that we are living in the same house together, etc. My question is, can they type this up, sign it and then do they need to have it notarized?

Also, can it be typed/printed or does it have to be (or should it be) handwritten? Thanks. I've used an attorney in the past for the I-130/I-129, but may try to do this on my own this time.

Can be either handwritten or typed.

The example one indicates notarized signature. It is a Sworn Affidavit which implies needing signing in front of an officer able to give oaths (a Notary).

http://www.visajourney.com/examples/USCIS_I751_Affidavit.doc

http://www.visajourney.com/examples/I-751_support.doc

oh, I didn't know it had to be a sworn statement. I just figured they can take their letter to any notary and say they want it "notarized." I guess this is different than having it "notarized"?

The action of notarizing is signing it in front of a notary officer and swearing that everything in the letter or document is the truth.