Use PG 7.4.x, no problems with OACS 5.1. You can run both PG versions on the same server, just use different ports, no need to compile them differently (though you should make sure that you use /usr/local/pg74 or something like that as your prefix for ./configure).
I'm fairly confident you will run into problems upgrading OpenACS from the 3.2.5 version though (but I think that is not your intention). Running 3.2.5 on PG 7.4 won't work most likely either, but you might be lucky (just try).
