I've done this for many presentations. Simon has the best and simplest way to do it. Just stack layers leaving bottom layers up.

Be careful though and do not use a black background as there might be transparency issues if your letters are anything but pure white. Instead render the images as PNG or TIFF with the background made of transparent pixels.

At one time Richard had mentioned creating a play mode which would work with index positions in Keynote so that a user could export a Keynote presentation as a Quicktime movie and then initiate a cue which would automatically halt at each index position.

To do it the way Simon mentioned, your best bet would be to just create the the movie with the freezes a specific distance apart (4 seconds for example).
In cue 1 of the list initiate play once forward with outframe set to 100 frames (100 = 4 seconds in 25fps). In cue 2 set outfram to 200 and the file will continue to play for 4 more seconds. Continue this etc.