You can easily make back-ups of your dvd's with DVD Decrypter. I would advise you download this now; I'm not going into details, just download it now as it won't be around much longer. It is also free.
It removes all types of CSS encryption methods; not positive about the newest kind that Sony has just implemented though...
While your at the addy above, you might also search around for DVD Shrink too. The two go hand-in-hand.
Shrink will compress the material down and allow most dvd's be fitted onto a 4.7 GB disk. You can also omit the extra's such as the director's commentary or foreign languages which take up quite some room.
DVD Shrink is also free too.
I don't know what model Memorex you have, so if yours is a dual layer and if you have the $$$$, you could also consider the newer dual layer media too. They are still quite expensive, though.
Happy burning... burn baby, burn....