i would think you should be able to play MP4's i guess just it depends on the resolution,

when downloading whole movies, if they are 700 mb's +/- it should play just fine, when you get into the HD stuff i think its more taxing,

in my case i have a old 1.6ghz pentium m, gateway laptop, running xp, i couldn't use VLC! VLC just would not display video, it would play the audio but you couldn't see anything.

i downloaded mplayer and it plays video flawlessly now,

http://www.mplayerhq.hu/design7/news.html

maybe it will work well for you, looks like they have a OSX binary!