if you want to print 2-3 megapixels could be enough for a regular sized photo I think. depends on the quality and sharpness you look forward to.
you can do the calculations yourself. for example:
4x6inches=24
300x300dpi=90000
24x90000=2160000
so that's ~2 megapixels for a 4x6 print at 300dpi. I don't think that printing services go with a resolution that high though.. not sure. I can tell you that I made a 5x7print from a 640x480 file at a konica studio and the photo came out really nice.
if you want to look the pics on the screen - 1600x1200 (2 megapixels) is pretty large. so a 2-3megapixel camera should fit your needs.
there are things perhaps more important than the resolution though - quality of the lens, optical zoom and features. so don't judge a camera just by the MP count.
hope this helps