I've been running large prints from the days of the D100/D2H, up to 24x36 on those and even larger with the D2x, D200, D300 and D700, up to 30x45 and larger prints I produce myself for my clients. Not is sophisticated to know the difference on any of the prints. When I show a 24x36 inch print made from a jpeg file from the diminutive 4 mp full-rez of the D2H, people are astonished since the math says that the maximum print size capable from that camera is barely 6x9 inches...
That said, if you look at a native file blown-up to any of these print sizes on the monitor, you'll tell the difference. But on paper at a proper viewing distance the issue is mum.
Basically, you can emulate this experiment within reason by comparing a D700 and a D300 using the same lens on both and then cropping the D700 image to the same field of view of the D300. It isn't perfect but it will give you a pretty good idea.
If you are simply doing prints up to 11x17, for instance, I don't think you'd see any difference even closely. Larger prints you may if you are super critical.
For image samples, check the Nikon site for sample images. In past announcements, they seem to post a few images with each new camera. Another place to look is at Nikonlinks.com. You may get lucky and someone has side-by-side images.
In my opinion, the files from the D700 are fantastic though the D300 are not too bad ether. Under marginal lighting and less-than-perfect conditions, the IQ edge goes to the D700 simply because of the larger photosites on the sensor. I don't hesitate to run the camera regularly at ISO 400, 800 or 1600 or even push the chip up to 6400 and 25,600. With the new noise reduction in Adobe Camera Raw and upresing from the raw file, it's hard to beat the Image Quality, I've found. It's opened a whole new world for me!
At lower ISO ratings, you may be better with the D3x. If you are shooting under marginal conditions regularly or need 720p video, go with the D3s and upres as needed. Depending upon the final use, IMO, you should be fine.