Roger, welcome to OpenACS!
Yes and no. The packages in 4.6 that constitute the "acs-core", i.e., the ones that get installed automatically upon the initial install, should work on 7.3. But you'll probably find that a lot of the other packages won't (due to differences in the timestamp datatype(s) between 7.2 and 7.3).
Since you are new to OpenACS and probably would like to check out some of the other packages, too, I'd recommend PG 7.2. That will certainly give a more stable impression.
Regarding the "Next" button, it may well sit there for 15 minutes depending on you hardware. I'd estimate it takes about 8-10 mins on my K6-3 450, though, before I get the Next button.
If you think something is wrong, drop/recreate your db and try again with "tail -f /path/to/aolserver/error.log" running on the side. If you spot an "Error" in there, post it here and we'll try to help you.
/Ola
