When are megapixels too much?

Hi, I'm interested in buying a new DSLR soon and I was intrigued by the Canon 5D Mark III and the Nikon D800. What interested me in the two is how many megapixels the Nikon had: 36.3!

This is what made me ask what if I'm wasting money on this and could've gotten something much cheaper like a Canon 60D? So with that, I ask when are megapixels too much?

When it reduces rather than improves the end result (such as poor low light performance), then it is too much.

You know you've got too many megapixels when they exceed either your wallet or your needs.
A 36 MP camera needs costly top-grade pro lenses, otherwise every minor optical error of cheaper (kit) lenses will show up in your images. The huge file sizes these cameras produce also implies that you'll need a high-end computer with lots of memory and storage to process them. In other words, you'll need to invest heavily in the camera system and image processing gear, not only in the camera body.
36 MP is nice if you want to make enlargements the size of a building. Whether you really need that, only you can decide. To help you decide I've included a link to Ken Rockwell's much quoted megapixel myth article below.

You certainly have too much megapixels in your hand if during your entire life you view the pictures in the tiny LCD of your camera only and not making prints or looking at them through the large monitor of your computer.