“Just 10 megapixels”
An update to 12 won’t make such a difference one could be mislead to believe.

Example, four Nikon cameras: Table 2

The difference from the 10 megapixels (MP) in the D40x to the 12 MP in the D300 is:
416 px horizontally and 256 vertically…
which is about 10% more info in both ways.

The step up from 6 MP to 10 MP makes a larger difference as that usually involves a change from CCD to CMOS type sensor, with lesser noise for the latter.

My advice: Choose camera based on other criteria than megapixels.
For example: Light sensitivity and noise; a noisier image is harder to handle, and noise follows with larger amount of pixels – as the pixels needs to be smaller. The tecnical background is that smaller electronics is more affacted by surrounding disturbances and internal physical facts – result: noise.

One more thing that that speaks for keeping the megapixel count down; one megapixel more is three megabyte more data to handle, double that (six megabytes!) if you shoot raw.
Stepping up may require updated computer hardware, to keep up with the change.

Printing then? Well, in general prints are real nice if you succeed to put 300 pixels per inch on your media (that is more than 10 pixels per millimeter).
Big prints, posters/placards can be printed at 125 pixels per inch (5 pixels per mm).
Which makes the sizes listed in the table available for you, printing smaller will increase quality – if your PRINTING DEVICE supports it. Note that I wrote “300 px/in on your media” – by that I mean that those pixels has to be visible by inspection, they have to physically exist on the output media.
Now note: It is quite common that a printer device uses a 6×6 matrix to represent a dot, then it has to print at 6 x 300 dpi – true 1800 dpi – for each color it puts on the paper. If it doesn’t, then the output is less than 300 ppi.

With this information, for high quality prints at 300 ppi – as can be seen from the Table below – the difference from 10 to 12 MP isn’t that big.  Just 1.4 inches (1.4×25.4 = 35mm) extra width and 0.9 inch (22mm) in height. And each step up makes a similar difference, one or maybe two inches.

If you print at smaller sizes, say normal photo’s (6×4″ or 15×10 cm) you won’t see much difference at all.

My conclusion from this: To make it wortwhile to update the camera body – either the megapixel count has to double (that is; double surface area on the sensor, factor 1.4 for height and width) or some other capability be drastically bettered, to something really tempting and thrilling.
That might be a radically better ISO performance, IF you do use that!

Also consider this fact: Up’ing the megapixels makes the flaws in your current lenses more visible – or at least turn it into a new limiting factor. you won’t be able to use the higher resolution fully until you have found replacements that match the body.

Pixels 125 ppi , as mm 300ppi , as mm MP Body
3008 x 2000 24.1 x 16.0 , 611 x 406 10.0 x 6.7 , 255 x 169 6.0 D40, D70s
3872 x 2592 31.0 x 20.7 , 787 x 527 12.9 x 8.6 , 328 x 219 10.0 D40x, D80
4288 x 2848 34.3 x 22.8 , 871 x 579 14.3 x 9.5 , 363 x 241 12.2 D300, D700
4928 x 3264 39.4 x 26.1 , 1001 x 663 16.4 x 10.9 , 417 x 276 16.2 D7000, D4
7360 x 4912 58.9 x 39.3 , 1495 x 998 24.5 x 16.4 , 623 x 415 36.2 D800(E)

edit: 2012-03-03, D800 added.

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