A couple small points you might consider-
1680x1050 and 1920x1200 are both native 16:10 monitor resolutions. So you're not going to have a different aspect ratio from your old 2209wa if you go 1920x1200, just a different pixel size/pitch. If you needed to, for legibility or video card reasons, you could just set the monitor in windows to 1680x1050 without any stretching.
My understanding is that Windows 7 does support wide gamut now. At least through (right click desktop) Screen Resolution -> advanced settings ->color management tab -> color management ->advanced you can change the color profile to a scRGB wide gamut profile.
Support for color depths of 30 and 48 bits is included, along with the wide color gamut scRGB (which for HDMI 1.3 can be converted and output as xvYCC). The video modes supported in Windows 7 are 16-bit sRGB, 24-bit sRGB, 30-bit sRGB, 30-bit with extended color gamut sRGB, and 48-bit scRGB.
So it might not be a showstopper. My impression is that since windows is the most commonly used platform, monitor makers aren't going to design them in a way that the most modern software can't handle properly. But since I don't have the sort of truly top tier level color accurate setup at home that you'd seem to prefer, I can't vouch for it 100%.
I don't know if you have a device like a Pantone Huey Pro calibrator, but such a device might interest you if you don't have one and really are going for dead on color accuracy. Especially if you did end up saving a bit through getting a different replacement via dell.
As for 6 bit + FRC vs 8 bit, I think that they do a good enough job for most people, especially those coming from TN's, but I don't think that there are any that truly hit every color spot on via FRC. You probably wouldn't notice it after a bit if you got readjusted to the new "standard." I know that some companies, like Samsung, get a bit tricky and do the signal processing at 9 bits to get more of the dark grey tones before the monitor outputs to the 6 bit + FRC. I'm sure most do their own tricks, but thats the one I see mentioned the most. The end result isn't quite as good though, which is why the more "premium" monitors are still 8 bit.
Anyways, good luck again, sounds like you have some options, and since a lot of these things are personal preference it will just depend on what you're exactly wanting. Still, thought I'd toss in the thoughts on the resolution/ gamut as it might widen your choices if either solves your issue with those specific points.