Am thinking of buying a external dvd writer help

Hi i'm thinking of getting a dvd writer however have no idea what to look for and how it all works. Can anyone give me any suggestions on what specs to look for and some good tutorials i could use.
Also i have extra question on how many movies i can fit onto one disk as i've brought copied movies before now that have about 3-4 movies on them and this is what i would like to do really can anyone help me figure this out.
So far i've been using my trusty cd writer but am fed up of having to split movies into two halves most the time.

anyway thanks for any replies
anxies

never Buy sony, get Samsung its a good one.
but if i had that much money to buy a Ext DVD writer i would prefer to Buy Yamaha Cdrw F1