I'm not familiar with dvdfab. However, if you are able to rip the audio seperately into its own mp3 file and a audio-less mp4 video file you should be able to join the two files using Yamb.
If the subsequent file has issues playing in your n900, you may need to run the subsequent file through handbrake.

When in handbrake make sure anamorphic is set to none and **** for DVD's the width is set no wider than 720pixels. Increasing the width for dvd's is pointless and a waste of space. Lowering the width will save file space at the expense of quality, its a personal choice.
Click the video tab, check constant quality. I prefer RF:16 to RF:15, again its another personal choice for space.

I guess I should point out that there are numerous places that provide instructions for dvd to avi conversion. Try videohelp.com or the search function of this site. The instructions I provided above are what I use to convert HD content to play on my n900, but I change the width to 800pixels.

I have placed a full movie, pixar's Up, on my n900 and it plays beautifully.