I use an 8 megapixel camera (Canon Digital Rebel XT), but that's probably overkill. I'd agree with Anthony that good lighting will give you much better pictures. Also using a macro lens or zoomed in telephoto will help you to not waste any of the pixels you do have available.
Maybe if you could link to some pictures that you think are problematic, some more advice could be given.
I'm trying to work up on article on photographing MOCs and posting them to Brickshelf (see my post here
in publishing). Unfortunately, I haven't dedicated as much time to it as I should.