I am currently using a Nikon d800. It produces a sharp file. The camera is a monster when it comes to capturing detail in the shadows--it never ceases to impress me. The files hold up well in heavy post. What I don't like about the d800 is the inelegant luminance response from about 200 up to 254. It's not subtle. The Sony a850 did a better job with highlights. As far as color, nothing beat the CF39-MS Hassey back that I've used. Those files are extraordinarily robust. The drawback with any MFDB system is the auto focus is not nearly as sophisticated, live focus is for tethered shooting in the studio (maybe the new Phase offerings manage okay in the field), forget about using an ISO setting two stops over base (the smaller sensor backs with micro lenses might stretch to 2.5 stops), and the lenses are expensive--especially when you head down the tech camera path using electronic shutters. But there is a certain look that a seven or eight-year old MFDB CCD chip offers. The midtones, highlights, rolloff, and color are better. The difference in sharpness between a P45+ and a Nikon d800 is moot. In fact, I've found that my pro Nikon lenses are far better than say the Hasselblad HC35, HC50 I, or either of the Mamiya 35mm AF or MF versions. The old Mamiya 50mm shift lens is pretty cool. The Mamiya 45 AF (non D) version is okay.
Every choice has it's pros and cons. For me, I need the versatility that a 35mm digital SLR offers. I've gone down the MFDB road before and love revisiting some of the files those systems produced--stuff that would not be possible with a 35mm dSLR (especially fine art reproduction). A very nice inexpensive camera that compares favorably to the old Aptus 22 back is the Sony a850. The full frame Sony is a great 24 MP camera, it has better than average dynamic range, terrific IQ, and the Zeiss glass is beautiful. It exhibits a smoother range of gradations in the highlights than the Nikon d800. The problem with the Alpha system is the lack of choices regarding lenses. The Zeiss 24-70 is special, and I've heard good things about the 16-35. I dropped Sony when it introduced the a99. That camera did not appeal to me on any level. I'll bet the image quality is about par with the a850 and a900 without the gorgeous optical finder. One nice thing about the a99 is that it has a very good live view capability whereas the previous FF Alphas didn't have live view.
As for Nikon lenses, the 14-24mm is in a class all of its own, especially in the studio where you are able to control lighting. The 24-70 f/2.8 is comparable to the Zeiss (the Nikon renders better sharpness along the far edges of the frame and the ACR lens profile corrects distortion and CA with a mere click), the Nikon 60mm macro lens is spectacular (far superior to the Sony), and the 70-200 f/4 VR is lightweight, portable, and optically superb; the AF is not zippy. I'd be curious to check out the new Nikon d610. My guess is that the tonal curve in the highlights is more nuanced than the d800.
Doug Peterson offers sound advice: test out a range of systems to see what works best for you. It's worth budgeting up to $1,000 to do that, because once you commit to a system, $1K is trivial.