It really depends on how big the video files are.
But to give you an idea, i've been watching the entire Juuni Kokuki anime series in my ds (around 28min each episode) and they normally turn into 55-65MB files.
As for movies they normally are 180-250mb big.
So to assume, you would be able to fit 2 (maybe 3) normal length movies into your 512MB card or around 7-9 tv show length episodes.