<< Well technically, there are two ways to do it. Not sure on the feasibility as I don't do DVD backups myself.
DVD Copy - This would entail making a bit perfect copy of your DVD to a DVD-R (or some variation there of). This is legal. You are allowed to make one backup copy of all media you own. And since you are making a bit perfect copy, you are not breaking any encryption. This copy would be identical to the original.
Hard drive copy - This would entail "ripping" your DVD to your computers hard drive. This is commonly done in the DIVX format for video and mp3 format for audio. Unfortunately, you lose the 5.1 surround sound when ripping and it reverts to 2 channel stereo. This is illegal as it requires you to break the encryption on the DVD to make the copy. According to the wonderful DMCA in the US, you cannot break encryption that is protecting copyrighted material. >>
Divx supports 5.1 Dolby sound, I do believe. You just have to make sure you have a program that supports it in the ripping of the 5.1 sound.