Just wanted to follow up on the SMP stability issue.
Disabling AGP sounded like a performance hit, so I read through the AMD issue report--which was awesome--and applied the patch to the 2.4.18-10 kernel, which RedHat released today.
So basically what I did was to grab "kernel-source-2.4.18-10" via up2date and apply the patch (see gbrauer's post for the link) to /usr/src/linux-2.4/arch/i386/kernel/setup.c before building a custom kernel. Red Hat's instructions were very clear although I encountered one error compiling the "transparent decompression extension" to the iso9660 filesystem. I disabled this option and was able to make bzImage/modules etc.
I rebooted into that kernel 30 minutes ago and haven't had a crash yet. During this time XMMS has been running that OpenGL plugin that quickly crashed my system under the stock 2.4.18-5 kernel.
Thanks again for the valuable information, hope this helps other people in the same situation.