If you are going to put your db on reiser in (which btw is what im doing) you are in essense doing double journaling. Which means for every write of the data to disk it require multiple head movements.
The approach I took in PG case was to create a separate partition for the log directory / XLOG and make this partition non journaling or ext2 in my case. You may want to symlink this and I would also recommend a separate platter or disk for the XLOG files. This allows the heads to remain ready at all times for sequential writes without having to move.
In general though if you are running a journaling database on top of a journaling filesystem I think it is beneficial to keep the database log on a separate non journaling filesystem and on its own platters.
Just my $0.02
