I have a 4MP camera and I usually do batch resampling to 640x480 for web postings. I noticed that the dimendions of 4MP are 2272x1704, which are not divisible by any integer to get to 640x480. I've read somewhere a long time ago that resampling down should always be by an integer factor, which makes the resampled image sharper because there's no interpolation involved.
For example, if 2272 was resampled down by a factor of 4 down to 568, then Photoshop would simply remove every 4 pixels. If it was resampled down to 640, the factor is 3.55, and the final image will have interpolation. I'm thinking the 568px image is better quality and/or sharper than the 640px one.
Now I read this 'integer recommendation' a long time, so I don't know if it's still valid or makes much of a difference especially with Photoshop's bicubic resampling.