I faced a similar problem with dvd backup softwares and eventually worked out a solution. See, when you compress a regular 8.5GB movie DVD to a single 4.7GB DVD, there will obviously be some quality loss. But if you really want the quality to remain almost the same, you wouldn't mind spending a li'l time ripping the DVD to Xvid/DivX, would you?
I tried DVDXcopy and CloneDVD before shifting to this much better way. On my X2 system, CloneDVD was replicating a DVD movie in ~18 minutes, but as you mentioned, the quality was far from satisfactory. So i did some research and found this guide: *www.rita.lt/guides/GKnot_DVDtoAVI.htm.
It uses GordianKnot ripping application with DVD Decrypter to rip any DVD movie to your choice of codec. I prefer Xvid but would experiment with x264 soon. You need to download GordianKnot and DVD Decrypter (google them) and then just follow the guide to backup a movie DVD. I prefer 2CD output i.e. movie output in two 700MB files. Not that i am gonna write those files in CDs but that setup works fine for me. The quality could be increased even more by selecting larger output file (i.e. lower compression = better quality.)
My system (AMD X2 with 2GBs of RAM) takes around 1 hour and 30 minutes to rip the DVD with the deinterlacing filter applied (most of the times.) Clearly, this does take a lot of time but i generally start the ripping late in the night and leave it overnight so it doesn't matter.
I'm sure this is the way to go if you want quality, otherwise compromise and have backups in around 20 minutes.
By the way, the guide may seem overwhelming the first time you use it but once you setup the codec(s), it'll be a matter of seconds to start a new ripping.