Can a 12mp camera look close to a 4x5 scan at 16x20 or should I not bother going there ?
It's been a couple decades since I shot 4x5, so I'm a bit out of touch with it, but my opinion would be that my first response would be "no, it wouldn't". However, with digital tonality is maintained into larger print sizes even when detail retrieval goes away, so whether you'd be happy with a 16x20 print from a 12mp DSLR is going to depend on quite a few factors such as your viewing distance, choice of paper, and of course your technique and the quality of the glass as well.
I'd say you might want to see if you can rent one (or a D3, since it has the same sensor) and see if it works for you.
My own thoughts are that properly done DSLR work with the 12mp or so DSLR bodies can achieve what MF film can do - I totally realize there are still many folks who might disagree but in my experience what digital has allowed me to do is go "up" one "film category size" (ie, from 35mm to MF) in terms of output quality while working with the smaller camera size. My own 16x20's that I print on an Epson 3800, when done correctly, are better than anything I ever shot and printed on/from 35mm film and at least match anything I ever did on MF. I'm not sure I can say that about 4x5 though. When done properly, there are some aspects of 4x5 quality that "come through" to well done 12mp+ digital work, but detail retrieval is not one of them - it's more the way the image holds together.
One thing to be aware of though - if you do hold out for the higher rez body, you'll have to have the very best glass for it - a Nikon wide prime from the 80's will not cut it at all - you'll very much have to look into investing in the very best possible lenses (which you might even find, at least in Nikon, sometimes is a zoom) in order to extract everything from a 20mp+ sensor. You'll be okay on a 12mp FF sensor for the most part, but if you go into the 20+ arena, the game changes.
Sorry for rambling - you really, I think, need to rent a D3/D700 and see how it goes for yourself.
-m