The I-134 is not a legally binding affidavit, it is used for non-immigrant visa's. Up until you marry (and subsequently file for Adjustment of status) the non-USC fiance will hold a non-immigrant visa. This allows them to get into the US before having a legal relationship with the USC fiance. The I-134 will need to be filled out by a qualified sponsor (the USC if he/she doesn't earn enough $ then also a co-sponsor) This form will need to be notarized. This does not get sent with the I-129F application. This is only submitted to the Embassy during the interview. Each Embassy will have information on this and will send it to you soon after your approved I-129F is forwarded to them.
The I-864 is a legally binding contract that the sponsor will be financially responsible for the immigrant in the event the immigrant attempts to benefit from certain federal aid programs. This form is filed at the Adjusment of Status part of the journey (after the Non-USC has travelled to the States and you two get married). I believe the new forms do not need to be notarized (check this) as they are already legally binding.
Hope this helps!
Sue
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