I use DivX player mobile, free download -
DivX Mobile
Then all you have to do is convert your movies to .avi or to Divx, i always use video size 320x240, as then the playback doesn't stutter, a movie will also be under 1GB.
Good converting programs are WinAVI, Xilisoft DVD ripper and ImTOO DVD Ripper, some DVD's have copyright protection, so use AnyDVD and CloneDVD to remove it.
I've got loads of movies on my
N95, The picture quality is excellent.
Hope this helps.