Hi T: The solution to your problem depends on several things: The size of your current sagging rafters; how they are attached at the ridge; how far apart they are (on-center spacing); the type and grade of wood and the pitch of the roof. Sistering on dimension lumber (2X6 is probably not enough) is a possibility, as is simply replacing the rafters one at a time. Trust me, if you remove the roofing materials, the bowed rafters will not spring back. Right now, they are in tension (as opposed to compression when they were new), and it will take a long time of applying static lift to straighten them, so jacking up the rafter is of no value.
I agree with Paul, if you don't have a good confortable feeling about taking this on, getting a residential structural engineer in at this point would probably be your best bet. If you're going to go through the trouble and expense of fixing this porblem, might as well do it right the first time :-)