The AOLserver situation's a bit muddy at the moment. The version most of us run is AOL3.2 + aD12 (arsdigita patch kit available at their website).
On the other hand, AOLserver 3.3.1 includes some memory leak fixes recently discovered and fixed by the user community. Rob Mayoff of ArsDigita is planning to release an ad13 patch kit for AOL3.3.1 shortly (AOLserver hasn't taken all of the aD bug fixes).
The bottom line is that AOL3.2+ad12 is probably your best bet for OpenACS 3.x - just run the Tcl 7.6 interpreter rather than the Tcl 8.3 interpreter (AOL 3.3.1 fixes a memory leak problem with Tcl 8.3 which many had never noticed due to using Tcl 7.6).
Don't let any of this make you nervous, we find AOL3.2+aD12 to be very stable (my server's been up 161 days as I write this).
The docs are slightly out of date, OpenACS 3.2.5 (due "daily" as Roberto slurps yet new fixes into the code base) will run under PG 7.1 (which itself was only release a week or so ago). OpenACS 3.2.4 mostly runs under PG 7.1, there are a handful of queries (literally "less than ten") that need massaging.
OpenACS 4.x will *only* run under PG 7.1 and better, so you might as well get started with PG 7.1.
