In terms of DVDSP, I've always used Compressor to make my MPEG-2s and AC3 files. I know DVDSP will compress a Quicktime file internally but I prefer bringing in files ready to go so the actual burn time is relatively short. FCPX will allow you to export to DVD but can it just export to MPEG-2+AC3 files without burning a disc?
Also, don't forget that Compressor will allow you to batch files, and attach actions to each (like upload to Youtube or FTP). That's the kind of work I like to let run overnight, so you can walk away from the computer and get some sleep!
JVK
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SCQT! Self-contained QuickTime ? pass it on!