Problems on DVD and cache
 

I got Vista RC1 on Centrino2130, Samsung X50, 1,5GB ram
If I start a DVD and choose the Media Center I got an decoder failure,
on Mediaplayer it says that there isn't enough ram to play a DVD
The Taskmanager is telling me that there are at all 1533MB physical ram,
about 1000 all the time are in cache and ~40 free...why there is so much in
the cache, how i can change this? The other~500mb are probably used by the
ati x700, how can I change this as well? I don't think that I need more than
600mb ram for the ati???
thank you,
chris