> we have already had one of her photos blown up to 2' x 3' and is displayed on a wall in her office.
OK, that's pretty different from most folks asking this question! That's 24x36 and while it's clearly within the range of what can be done on 12mp, it's definitely getting toward the outer limit.
> We are planning to do some much larger photos (3 x 5 or a bit larger for certain spots in our home) in the future.
Wow, 36 x 60 or larger. This will take some doing - but note that it is possible. Probably this - and certainly anything much larger - will need to be a multi-shot kind of operation, at least given 12mp or even 16mp (D7k).
> She also takes pictures of music bands on various stages in Southern California.
I suppose that things vary, but this is a fairly tough environment, technically. It's one place where the high ISO capability is used. And in sports I've been known to use ISO 1600 in full daylight, for various reasons, so night sports or less favorable conditions drive up the ISO requirements. (I'm simplifying a lot here, but the conditions under which I need 1600 in daylight have to do with credential limitations and the like, not technical problems. I certainly don't need ISO 1600 for an f/2.8 lens in full daylight.)
Your first request sounded very typical. This clarification clearly puts you in a different category than most potential upgraders. From the wide variety of work (landscape, macro, architecture, stage, sports), to the rapid progression to prints that are not just large but VERY large, and what appears to be an intense drive to learn and improve from the whole family, you may not be in the position that you need to upgrade - but you might be very close. If there are two or three of you, and your wife is really as into it as it appears, you could be needing at least two, very easily three and possibly four bodies with associated setups.
Still, my original advise about tripods and lenses remains valid, especially the tripod. A D7k does sound like it may be in your future. (You could be in for some serious investment, from the sound of it!)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!