Hi Mel,
In a nutshell, I think no. If you are contemplating buying a wide format printer to make huge banner prints, then it's probably worth it, but for the print sizes you mention, a modest increase could be helpful, especially with printing cropped images.
It is inherent that if you change your camera for an up-to-date version, you will get more pixels.
I'm in the process of  upgrading my old 8 MPx Canon to a new one with 18Mpx; the raw file size is tripled. So, in addition to deciding on the camera, it's necessary to think about the extra disc space required for image storage and the performance of your PC for processing. I downloaded some sample images to check out these points.
Rather than spend more than necessary on more pixels, consider spending on better quality lens(es); that will show more improvement to your images and prints than a few extra pixels.

We tend to think of QU being very good for making excellent prints from relatively small (pixel size) images; the opposite is true too, it's very good at making "small" prints from large images.
The same world class re-sizing algorithms are used with the bonus of user adjustable anti-aliasing.
Terry

Ernst,
Would you give a direct link to this information please, I cannot find it on the web site.
Thanks,
Terry

Using both a D700 and a D800 I will comment as follows:
For prints up to A3 my D700 (12Mp) is brilliant. For prints at A2 and above the D800 comes into its own.
I strongly endorse the comment regarding the quality of the lens; an excellent lens on 12Mp trumps an average lens on 36Mp, regardless of the size of the print. Photographic technique also plays an important part in the race for quality, and more pixels is more unforgiving - a blurry hand held picture is just as much rubbish taken with 36Mp as with 12 Mp (unless the blur is rendered with artistic intent). Good technique is essential when using high megapixel count.
One advantage of the high pixel count (when used in conjunction with good glass etc.) is the ability to crop more aggresively; you can almost do away with using the camera in portrait format and just crop the landscape one to suit.

Thank you all very much - you are reinforcing my thinking almost exactly.  I would rather spend the money on glass rather than sensor size, but I am considering going to an FX camera and probably will at least wait to see how the upcoming D600 stacks up.  Will also check out the DxO sensor rankings.

It's funny - I was not an early adapter to digital.  Ten years ago I was still using my clunky Mamiya TLR 6x6 format and I scoffed at the idea of digital.  I can remember doing a calculation - based on grain size and pattern - but am sure it was completely erroneous - that convinced me that I would need at least 50 MP to equal the quality of my 6x6 or 6x7 cameras.  But that was before Qimage.  I now look back at some of those old pictures - still love the quality of them - but remember not too fondly the many darkroom hours sometimes needed to create just one good print. (And wait till it's dry, and oh my, is that a blue cast or cyan, and how much?)

Mel W.  Columbia, Md.

Don't overlook the often ignored prime lenses - sharp as a tack and much cheaper than the equivalent quality zoom. In addition, every prime lens comes equipped with an organic zoom mechanism - your legs.