There's at least one recent thread about Oracle v. Postgres (this Q&A forum? the aD forum?), so it's probably worth checking that out. I stopped doing web/db stuff a while ago but I did set up an OpenACS site as a "proof-of-concept" recently and installing Postgres is *much* easier than installing Oracle.
Installing OpenACS (AOLserver, Postgres, OpenACS) took me an afternoon (and I haven't touched the ACS for over a year) so in answer to your question about where to start, I suggest that you start by downloading the software and following the install guide. If you've installed Classic ACS before, it's pretty straightforward especially since you're at the "just dabbling" stage.
