All else equal (it never is), the size that can be printed with 5Mp vs 3Mp is one of those rare things that can be exactly quanitfied. The increase in size is the square root of the ratio of pixel count: 29.099..% in this case. That is a real increase, though less than going from a 5x7" to 8x10" prints. Close to, but still less than going from 8x10" to 11x17".
That exactitude does not answer the perennial question, "How large a print can I make?" Most folks agree with Slipe that 8x10" prints can be made from a 3Mpixel image. I generally agree, though IMHO consumer 3Mp images of subjects with lots of detail don't work well at 8x10".
All is not equal though. In general, as pixel count increases so does noise. Even if the noise is still not noticable, it will limit usefull ISO and not work as well at very long exposures. There are other issues with cameras that can easily be more important than the difference in pixel count.