Hey Wizard,
Most here on the forum will probably recommend either Sibelius or Finale for the notation software route.  Myself, I've been using Notion 4 which to my knowledge is the only one that currently works with Sonar in Rewire 64 mode. The track info translates fairly well from Sonar to Notion with an occasional scoring glitch in the time I've spent with it.
Export/import Midi might be a more sure or safe way.
Best,
HeyDan 

Just to expand a bit on what's already been posted: Rewire is an audio link; it will get the audio generated by score-driven instruments hosted in the notation program back into SONAR for realtime playback and recording in sync with SONAR's clock. But it won't allow you to have the score drive instruments hosted by SONAR or facilitate transferring the MIDI to SONAR.
 
I also use Notion, but not extensively. It has its own orchestral sounds and can use any VSTs you have that are available to all hosts. But if you want to work with the MIDI in SONAR, you'll eventually have to export/import MIDI or XML.
 
Incidentally, my main issue with Notion is that it's limited to 44.1kHz, and I've been running SONAR at 48kHz for a long time now which makes it inconvenient to work with Notion.

Thanks everyone, covers all the information I need to know. It's such a shame that sonar doesn't have a better built in version.

Cheers

Bibs

lawp
just to add, rewire protocol does support (4080!) midi channels, though it may not be implemented by individual apps http://en.wikipedia.org/wiki/Rewire

Thanks for the correction, Paul.  I drew that erroneous conclusion from the lack of information I found on MIDI streaming when I originally looked into it. But maybe it's just Notion that doesn't support it.. or i just didn't look hard enough. I'll have to check it out.