Let me make notes as follows:
1. The hardware is IBM 814334G upgraded.
Do you heard about any Open BSD bugs for the above hardware?
2. The both, X and Win GUI, are not vitaly required for Moodle server only.
3. Yes, Solaris is a very good choice, however
here Soloris for PC is not used in practice.
4. Yes, just tuning on the operating system (OS) is not enough,
however it is a key decision. The OS tuning is a spare power
for other decisions (httpd server or RDBMS server for example).
There are various ways to optimize the application itself.
I do not like to reinvent the wheel, i.e.
the server modules and opportunities are wellcome,
incl. SQL knowledge and programming in depth.
5. In the case of production Moodle server,
I prefer Apache as a more traditional and proven choice today.
Meanhile, Open BSD is a traditional and proven choice too.
6. The major requirement is the server to work non-stop,
i.e. reliability about an IBM hardware,
an operating system, a web server, and a RDBMS server.
7. If you have spare time only then please look at
http://www.lethargy.org/~jesus/misc/BBPostgres.pdf
P.S. The both projects, BSD and PostgreSQl have the same origin.