Wow, that sounds like a hell of a mess!

Someone's about to fall through that floor. Sounds like no parties at Katydid's place for a while :-(

This house, if I remember, is circa 1840 right? ...are the timbers sawn or hewn? You say the joists pulled away from the girder. By girder are you talking center of the building or exterior wall? Is the exterior wall bowing out? That will want to be brought back in if possible (you probably know that anyway).

Those joist repairs sound pretty lousy...did you say 1X4 sistered to one of them (head shaking)? Sounds like you had a hack in there before you, which definately makes it harder for you now.

Are you in any historic area of town?

Do you want exposed original beams/joists? Do the floorboards have character?

If you keep the original framing, you could save money and time in not re-doing / drywalling a new celing. (But you said the furnace ducts are there...not exactly something to show off... perhaps the ducts can be re-routed in the wall or some other area like a closet?)

I'm sure it's tempting to rip it all out, but that may not be the cheapest in the long run. Any broken joists can be replaced w/ reclaimed stuff sawn to size...

Fun fun