In fact the "effective pixels" too require some interpolation. The CCD's pixels are monochrome ones, so of the total number of pixels 1/4 are senzitive only to red, 1/4 to blue and 1/2 to green (green is doubled because the human eye is also more sensitive to green than to other colours). So a 3.2 megapixel camera has 800,000 red, 800,000 blue and 1,600,000 green pixels. After taking the shot the camera's software interpolates the data and produces a 3.2 megapixel RGB image. Now imagine that the built-in program performs another interpolation and produces a 6.4 megapixel image from that. Weird, isn't it? But the "6.4 megapixel (interpolated)" specification looks better than the bare "3.2 megapixel".
Besides resolution there are a lot of specifications that worth comparing when buying a digital camera: auto and manual focus possibility, program and manual exposure, file format, ISO range, max and min exposure time, lens aperture, lens manufacturer, etc., etc.
I saw a 4 megapixel Fuji Finepix that produced worst images than my entry level 2.3 megapixel HP Photosmart, so resolution is not the main issue...