Well welcome to the community

may your ppc bring you all the joy it has brought us!
If you really want to know more about movies on ppc reading through the links that Crystal and Prismejon gave is a good start.
To answer your question about minimums, to watch a movie on ppc you'll need:
-a Pocketpc (but you already knew that didn't you!?! :wink: )
-a player: PocketMVP(Divx, mpeg) or PocketTV(mpeg)
-a 128MB Compact Flash or Secure Digital memory card. Like Prismejon said a 512MB or 256MB card is preferable but 128MB is the bare minimum you can get away with for a 90minutes DivX movie. All depends on what you want to spend on it.
As a rule-of-thumb you should count 150 to 200MB for a 90minutes DivX movie (it is always possible to do more ..or less but it's always a two-edged sword (more quality->bigger size / smaller size->less quality) Should you want to go with mpeg instead of DivX (which i don't recommend) the space required for the same quality, will be higher.
If money's no problem get a 512MB card otherwise get a 256MB and if you're really on a budget 128MB
Once you start reading about it you'll soon notice that everybody has kind of their own ways of doing things + personal settings and this affects the the final filesize greatly. Experimenting will be the best way to find what quality versus size suits you the best. The best way to do this is by using short clips to experiment, so you don't have to wait for a whole movie to encode each time.
Have fun.