I use The KMPlayer http://www.kmplayer.com/forums/showthread.php?t=12142
After using countless media players over the past ten years, I can’t recommend this one enough. A word of warning though: it has so many options it may confuse those less technically-minded. I really don’t mean that as an insult, but it can seem overwhelming at first (just right click on its window and you’ll see what I mean).
I’ve just got my blue nc10 (still 1gig of ram) and have been playing 720p quicktime and wmv trailers (off gametrailers) no problem. 1080p quicktime movie trailers have audio sync problems though, so I wouldn’t recommend it. I’ve played MKVs but not HD res.
Hope that helps.