The sensors in consumer digital cameras are generally the same size in physical dimensions, when you cram in more pixels, you decrease the actual size of the individual photosites (pixels). The smaller the photosite (pixel) the harder it is for it to gather light, to compensate for this they have to up the amplification which results in harsher and harsher noise levels as the pixel number increases.

Apart from this, you need to quadruple the number of pixels to double the resolution. A jump from 6 to 8mp is only a 25% increase which sounds a lot but is really marginal when it comes to final print sizes. To double the resolution of a 6mp camera, you need to go to 24mp!! So for you hard earned bucks your not getting a whole lot, its pretty much a marketing gimmick. If you aren't printing bigger than A3, 6mp is more than enough. My 6mp D70 made all the photos for my first exhibition, the print sizes were about 16"x10" and the quality was superb. For larger than standard 5"x7" prints, 2-3mp is perfect.