I would expect that product pages for each player would say what operating systems the product is compatible with.
As far as landscape mode, because many media players (including BetaPlayer) support video in landscape mode on WM2003 (note that landscape mode is not "native" to WM2003), that they would also support landscape on PPC2002.
For links to the product pages, see the media player listing at
http://www.feederreader.com/mediaplayers.html
Greg Smith
Author, FeederReader - The Pocket PC RSS, podcatcher, videocatcher
www.FeederReader.com - Download on the Road