Well, there are a lot of good ideas posted here so far. I like the Machka rule of 1/2 my body weight as a guideline.
Aside from the weight issue I believe that you also need to think about the space required to carry your stuff. Try to gather up all of the stuff that you feel you just can't do without and figure out the size of panniers that you will need. Be sure not to get panniers that are larger than you need because you will end up filling them up with stuff you don't need.
I understand that this discussion can quickly diverge to other issues, but one of the reasons that I have never tried a trailer is that I would have very little space restriction for packing and would end up carrying too much stuff. I could mount four panniers (I always carry two panniers), but then I would have space to fill up with stuff that I reallly don't need.
The space issue also enters into the decision on which tent and sleeping bag to buy. Look carefully at the "packed size" listed for these items when shopping and determine if they will fit into the panniers that you are considering. Some tents pack into reasonably small diameter tubes but they are long and your panniers need to be deep to fit them in. Unless of course you intend to carry the tent and bag on top of a rack. Good luck with your planning.