FreeBSD is *very* stable, and if you've been reading on a few stories lately on Slashdot in the past week or so, its performance in most cases is the same, if not better than Linux 2.6. FreeBSD's TCP/IP stack is considered one of the best (if not the best) implementation. This may not, however, be all that important for OpenACS.
Postgres runs like a charm on all of my BSD machines, and I had very little trouble setting up OpenACS (aside from the known little patches to get a few models compile). This, unfortunately, may change with AOL Server 4 -- the stock distribution does not compile at all -- there seems to be just too much Linux-ism (no flames, please) in it (NB: fi your experience differs -- please let me know. I think I saw a few posts mentioning AS4 on OSX which is FreeBSD with Mach kernel).
FreeBSD does not have journaled FS, but it has a feature that maybe better in some circumstanese -- soft updates. I am not sure, however, how it affects Postgres -- never benchmarked it nor seen any benchmarks on it.
