Yeah, I've been pondering the choice of sensor and system. Regardless of which dSLR body I get, my old Mamiya/Sekor threaded lenses won't be very useful, as they are the SX line without an easy way to shift to manual aperture operation (unless I can figure out a way to lock the lenses in manual mode or have them modded, but it would probably be easier to just pick up some more suitable lenses). I guess I should say, they could only shoot wide open, unless modded.
I like the 4/3rds concept (more "square" aspect and expectation of multiple formats and video) but there doesn't seem to be a lot of commitment to dSLRs with optical finders (both Panasonic and Olympus are focused on mirror-less). Also, it's half the size of 35mm, which does have a little effect on depth of field.
APS-C is ubiquitous and lots of cheap options. These days, though, full-frame might be within reach in the form of a used EOS 5D body (maybe even MK II).
Not sure which way I'll go, but anything will offer some better options for some things I want to do, like portraits of kids and photos at their activities and games. I push the limits of my G3, but the auto-focus is often too slow (and its manual mode is only useful for stationary objects, not kids), it has some limitations in low light and can't really do distant action shots well, all of which a dSLR can improve on.
Cheers,
Otto