There are a few cameras that can embed the date/time in the picture.
I don't know of any that can do this at 640x480.
I believe all of the Canon SD series of cameras can embed the date/time.
But only at what they call postcard size (1600x1200).
You could take all of the pictures at 1600x1200 and then do a batch resize to 640x480.
A free program called Irfanview can do a batch resize.