i had bought a 512 mb and i am wondering how much files it can store and it also depends on file size like if i put a dvd movie on my 512 mb will it still store alot of files?

I only have 1GB cards, but to give you an idea:

a) 6 45 minutes episodes of Gerry Anderson's classic "UFO" tv-series (converted to 770 format, 118-ish MB each)
b) 3 CDs, admittedly encoded at a low bitrate of about 35 MB each
c) around 150 e-books (I read a lot)

still leave me a glorious 17 MB on my 1GB card.

If you resist the temptation to select the highest quality of conversion(which I think doesn't add enough on this device to make it worth while), a 1GB card will provide you with 6 hours of video, over 24 hours of music or a lifetime's worth of books.

(Disclaimer: Author does not know your actual lifetime)