The exact figures are 4704 x 3136 = 14.75 megapixels. That's exactly how many pixels you will see on a computer screen. But, those are pristine pixels, not the interpolated "fudged" pixels like in normal cameras. You can compare Sigma Foveon resolutions with other cameras by multiplying the number of megapixels by some number. Here are the numbers I use:
Top-end Bayer filter cameras with top-end lenses:
14.75 megapixels x 1.5 = 22.05 megapixels, compared to Nikon D800E at 36.15 megapixels.
Mid-level Bayer filter cameras with top-end lenses:
14.75 megapixels x 2 = 29.4 megapixels, compared to an average semi-pro quality SLR.
Enthusiast-level Bayer filter cameras:
14.75 megapixels x 3 = 44.1 megapixels, compared to an average enthusiast compact camera like a Canon S100.
Basically, the Sigma Merrill compacts are better than many pro and intermediate SLR cameras, partly because of the fixed lens. That's only true as long as you're working within the narrow limitations of the Foveon sensor with no high ISO, leisurely slow photography, etc.
If you already have a Nikon D3100 and a FujiFilm X100, I think you will be impressed by the detail captured by the compact Sigma DP1 Merrill and DP2 Merrill cameras. The X100 is a good camera, but it's not in the same league. Only your Nikon D3100 will be able to compare respectably well with the Sigmas. They say the Sigmas don't do well at high ISO, but at ISO 800, they look better to me than the D3100.