Postgresql runs one process per client, so it should take advantage of a multi-processor setup; likewise AOLServer's multithreaded architecture.
The AMD multi-processor architecture has seperate 2.1 GB/s bus per processor; Intel use a single 3.2 GB/s bus shared amongst processors. Which works best will depend on the application, but I'm guessing that for a web/database app aggregate bandwidth is most important (giving AMD the edge as long as you use fast enough RAM and good I/O cards).
Of course, you need to be sure your disk I/O isn't actually your limiting factor.
Speaking of cheap interesting hardware for building solutions with, people may like to take a look at the Promise IDE RAID cases (http://www.promise.com/Products/UltraTrak/UltraTrak100%20TX4%20&%20TX8%20Data%20Sheet.pdf) - up to 8 cheap IDE drives in an external RAID case (hot swap bays and so forth), and an UltraSCSI interface on the back of the box. Cheap RAID with a sensible interface to the server.
