Hi Joe
Voice looks great, by the way! We moved from AOLserver to Naviserver about 5 years ago, and it really was a pretty smooth transition. I highly recommend doing it, as Naviserver has so many improvements. We were on Windows but it should be much easier on Linux.
Like you we were on an old OpenACS fork and in the end after much debate, we decided against upgrading OpenACS itself.
We use Oracle, so I can't really comment on the Postgres side of things.
Brian