Ask 10 people get 100 answers ;)
Jon is being conservative. You traffic isn't going to be really effected by your parition sizes. Except for where you place your log files, depending on your application you could even turn off logging for performance reasons. Probably where you stick your partitions is more important. How you mount paritions, if your server is connected to the internet.. then you can also mount certain partitions read only or no devices or no executables depending on security issues.
Sticking your files that are being served and your DB on faster drives such as SCSI gives a boost in performance. I have heard the arguments that IDE is just as fast as SCSI... but my expericence has shown than SCSI is definately more efficient with the controllers, drives, and applications I have tested. Place the files and DB on a RAID mirror. (BTW software RAID usually outperforms hardware RAID if you have a reasonable CPU -- except for very expensive hardware RAID cards).
Alot of this stuff you change to suit your needs. Just expect to reinstall your box a couple times before going live and afterwards to keep up with security issues. Your own benchmarks will tell you what you need to do. You really have to do your own testing when you get sites upto the point where you are tuning it. I have seen hardware RAID card performance vary alot with applications. IBM had a card that didn't do well with the through put tests of many labs but if you had an application that was causing more erratic drive head movements with smaller data reads its caching algo was much better and then provided a better through put. If you were streaming out MPEG videos from a video server... you were better off with something else.
Make sure you have a good backup and recovery plan that is *TESTED*. If recovery is simple... it makes also changes to your partitions/hardware later easy.
DISCLAIMER: I haven't deployed a Postgresql/AOLserver/OpenACS site yet. But have had experience with applications that require heavy network and disk usage.
