this is kind of a touchy situation. remember safety first. keep in mind that you will be lifting a substantial percentage of the weight of your old house. while your beams (girders) could probably sag to the floor and never break, if a jack kicked out while raising the beam the sudden drop could cause it to snap. and guess where you are standing. i would suggest having someone there who has a clue about construction principles. furthermore w/o knowing the size and weight of what you are lifting i would suggest overkill with safety bracing.
that having been said, here's my answer to your specific questions based on codes in my area. to truly do it right, a three foot by three foot pad approximately 8-12 inches thick should be poured where you will be placing each post. resist the temptation, to put the post on the existing floor because you have no idea how thick the concrete is. give the pads at least 24 hours to dry before putting any weight on them.
figuring out how high to raise the beam is pretty easy. string a line across the bottom of the beam. pull it nice and tight. it is important to make sure the line is not hung up anywhere (that is easy to overlook with all the other things you'll be thinking about). now that you know how much the beam is sagging, you are ready to start raising it. as far as worrying about the drywall joints, there is an old saying that i think goes something like this: you have to break a few eggs to make an omelet. in other words, i feel that a structurally sound, level house is pretty important. any cracks in the drywall joints are easy to repair. but that is your call.
depending on the length of the beam, you may need more than one jack. take your time. raise the beam a few fractions of an inch at a time, then rebrace. always keep your eyes and ears open, this is serious stuff.
once you get the beam where you want it (if possible, crown the beam slightly because it will settle slightly when you are done)place and adjust your posts. you can beef up the beam by adding more lumber at this time. make sure the ends of the new lumber are over top of the posts and after nailing it well to the existing beam, throw a couple of lags in for good measure. ease the beam down onto the post and that's about it.
oh yeah, go upstairs and run some water in the tub. its nice when it's not overflowing on one end and only half full at the other.
hope i helped,