Issue: Control the amount of memory used by the kernel
In 32-bit linux, you'll have noticeable performance problems if the server has more than 24GB RAM (it sounds backwards, but that's right -- more than 24GB is bad). This article describes how to limit what the kernel has access to so it doesn't exhibit the performance degradation.
In /etc/grub.conf, you can limit the RAM that the OS will try to use by adding
mem=<some number of mb>M
for the kernel you're using, like this:
title CentOS (2.6.18-194.11.1.el5PAE)
kernel /vmlinuz-2.6.18-194.11.1.el5PAE ro root=/dev/VolGroup00/LogVol00 ide=nodma mem=16000M