iDvd likes lots of elbow space. So hopefully that 20 GB's you mentioned above does the trick but if not ... things to consider removing for more HDD space from your boot volume / HDD would be items like completed iMovie and iDvd project files. If you have a vast collection of photos & videos; burning them to dvd helps as does using Monolingual to clear languages other than English (if you can afford to do so). The latter will clear about 500 MB's from the main HD .....
http://www.jklstudios.com/misc/monolingual.html
Btw, please use caution when running Monolingual. If you opt to download and run Monolingual, and delete unneeded items from the /Library/Printers/, /Library/Packages/, and /Library/Updates/ folders then DO NOT use Monolingual to delete architectures on any Intel Mac. (you'll thank me later and so will your keyboard).
Hope this helps but if not just come on back.
Message was edited by: SDMacuser