Copying DVD

My PC spec is a pentium 4 processor, 2.79GHx, 504Mb RAM, 80Gb hard drive.

I am trying to copy DVD films using my DVD-RW drive (think it’s 16x), but only seem to be able to copy onto about 5 separate DVDs.

1. Should I be using some sort of file compression software (using Nero OEM suite).
2. Do I have to copy all the files on the original DVD, and if not, how do I identify which files to discard.
3. Anything else I ought to be doing ?

Thanks.