You could probably get by with a Pentium I but 16 MB of RAM won't handle it. I'd guess you need at least 64 MB. I ran an OpenACS 3.2.5 site on a Pentium I class motherboard with a K6/2-400 MHz upgrade and 128 MB of RAM for a long time and it's performance was perfectly adequate. OpenACS 4.5 needs more RAM (depending on the number of packages you have installed).
--cro