Thanks very much for your message - it helped me make the disk! I changed the settings in the drawer from "audio" to "mp3" and suddenly the amount of memory shrank back down.
I didn't get an "out of memory error", it just told me there was not enough space on the disk when I hit the burn button. And I was trying to burn onto a disk that I could play on my car stereo, which doesn't accept all disks but I'm not sure what its specifications are. I wasnt' using the mp3 setting - I had selected the "audio" tab, and under "format" in the sidebar i'd ticked the button marked "audio cd". But I got the disk I needed in the end.
Great news, but if anyone can spare the time to talk me slowly through it, I'd still love to know what toast is actually doing to the disk when you choose "audio" instead of "mp3".
One more question: if I drag a whole bunch of files from itunes into toast, and set it to "mp3", do they burn as mp3s, regardless of the fact that they started out as mp4 or wav files?
ONce again many thanks for helping me burn that disk!!