Hello,
due to in-camera downsampling of the 8 MP image,
the noise level will be slightly reduced, and of course
the amount of purple fringing pixels, too (probably by
around 80%, taking the ratio of the image widths of
3264 pixels @ 8 MP and 2592 pixels @ 5 MP into account).
Personally I have ordered an F828 in the meantime,
as I figured that the "problems" of image noise and
purple fringing quite easily can be corrected in software
to an extent that fits my quality needs.
However, it is "clear" (so to say...) that the F828's
5 MP images will show more detail than the F717 ones.
You may want to take a look at the test on dpreview.com:
http://www.dpreview.com/reviews/sonydscf828/page17.asp