Yes, Mediamonkey will do teh above. You can set the N900 as a generic USB or WDM device. It works well unless you use WMA in which case the N900 does not see the track order for albums so all the track are listed alphabetically

Mediamonley also lets you decide at what point you want to limit the music at and you can do this by file type. I have mine set so that all the music transferred at over 192kbs is converted to 192 VBR MP3s. Anything under 192 is left as is. It can take a few hours when syncing, depending on how many tracks, but I sync to the micrsd card in a card reader so I do not have to have my N900 teathered to the PC whilst the sync takes place.

I considered all other options but none would do the conversion on the fly.

The other alternative was to re-rip my collection at a lower bit rate and save it elsewhere just for the N900, not what i wanted.