If you have Pro Res, you must have FCStudio, correct?
Compressor comes with an excellent Preset for YouTube.
YouTube also keeps an updated page on recommendations:
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=132460
http://www.google.com/support/youtube/bin/answer.py?&answer=165543
You may want to also check out the 'cow Compress Techniques forum:
http://forums.creativecow.net/compressiontechniques
~Michael
.: michael kammes mpse
.: senior applications editor . post workflow consultant
.: audio specialist . act fcp . acsr
.: michaelkammes.com