if you'll be using the ipad video app, you would have to rip and convert dvd to mp4 format. there is a free program that is reliable called Handbrake,you can just google search this one.
Another option is to stream the video from your desktop/laptop using an app like AirVideo.
I'm amazed at how well they work and it does save a lot of space on the iPad.
I just tired the air video last night. And it's nice. I have 16 gb wifi so just a couple movies will run all of my space. U do save a lot with the app.