Another vote for glass.
I'm also an owner of a D80 and I find nothing wrong with the camera. I've gotten myself the battery grip and an SB-600 for Christmas and I love the new creative freedoms I've gotten with the new flash.
Current-gen cameras as better than the D80 and D200 in low light, but I agree that it's better to simply buy lenses with larger apertures rather than being forced to use ISO 3,200, because you need to use aperture f/5.6.
So unless you really, really want to get into video, I wouldn't upgrade the body at all. Since you mention available light, have a look at Sigma's 30 mm f/1.4. Although the performance wide-open is not spectacular, there is no other lens like it (unless you're willing to spend a lot, lot more and find one of the rare 28 mm f/1.4 Nikkors). I've also been thinking about wide-angle options and Tokina's 17 mm springs to mind if you are interested in a wide-angle lens as it corresponds to 25 mm on full frame. For portraits, have a look at the nifty-fifty and the 85 mm f/1.8. Sigma's 50 mm f/1.4 lens is -- according to many reviews -- actually the best of the whole lot and beats Nikon's and Canon's older 50 mm designs. It's about on par with Nikon's new 50 mm AF-S. Alternatively, have a look at some of the macro lenses of similar focal length, they are well suited for portraits and you can do new things with them.
If you want a zoom, the best is certainly Nikon's 17-55 mm, but it ain't cheap. If you want something a lot cheaper, have a look at Tamron's 17-50 mm f/2.8.