IMHO the first thing to consider is where/when do you take pictures. If you are already carrying a bag, then the problems with portability is no longer a problem, you just need a better bag.
In the case of the new equipment, although you already know you want a Sony, have you considered the Pentax K10d? it seems like you already have a few lens, you can use those with no problem and then move to new lenses slowly. The K10d has several advantages compared to other dSLRs and the price is very reasonable.