Olympus Stylus 790 SW reviews:
http://www.imagingresource.com/PRODS/OS790SW/OS790SWA.HTM
http://www.cameralabs.com/reviews/Olympus_Stylus_790_SW/
http://www.digitalcamerainfo.com/content/Olympus-Stylus-790SW-Digital-Camera-Review-15847.htm
http://www.megapixel.net/reviews/oly-stylus790sw/790sw-gen.php
http://www.steves-digicams.com/2007_reviews/stylus790.html
http://www.thinkcamera.com/news/article/mps/UAN/665/v/3/sp/
As for the 850 SW, it's too new and there are still no available reviews (at least not good enough).
Anyway, the 850 SW model doesn't seem to add anything interesting in terms of additional photographic quality -or, for that matter, even operative qualities.
So, if the 850 SW is that much more expensive, I'd definitely suggest going for the 790 SW model.