Contributors often ask this question: "do the extra pixels matter?". Here are a few thoughts about it.
What values most to us and the buyers is the content of your image. If the aesthetics are bad, it doesn't matter what your camera can do or how many MP it can produce. This is part of the review criteria and is also taken into consideration at sales level (a very good small image can sell at a higher price than a not-so-popular one with more MP).
Please consider the following, after the above is acomplished:
First, as the number of megapixels increases so is the technology. That equals a more powerful camera, hence better images from a technical point of view.
Second, one can crop an image in a better way if there are plenty of MP, so you will have a creative advantage. Can equal less postprocessing time, or being closer to your subject without the expense of a telephoto. Of course, you will end up with less MP, but with a better image.
Third reason is that the price for larger sizes is better, hence contributors can receive better royalties. Even if we take into account the number of buyers who license for online usage, the average revenue per download will still increase when shooting with a higher MP camera.
Upgrade if you afford the price. My number one priority however would remain the lense and its quality. I would invest into that first and only afterwards in a better MP. The camera will be upgraded once more in a year or two, while the good glass will stay with you for many cameras to come.