Hi
If you look at this posting, they are still recommending postgres 7.2.3 or 7.2.4 with OpenACS 4.6.2. If you want to use 7.3, there is a bug in 7.3.1 and they recommend using a later version of 7.3.
There is an excellent guide on how to upgrade OpenACS versions at http://aufrecht.org/openacs-4.6-quick-guide/ch03s02.html