Hello everyone, through the past month or so of use I've stumbled across a couple of interesting tricks to tweak some extra speed out of my ancient iBook with FC4 on it. Hopefully some of these will help you as well.
Using Direct Rendering in xorg:
1.) By default DRI is loaded as a module, but the default depth of xorg is set to 24 bpp which if you're running that at 1024x768 is too high to load into RAM for direct rendering-- thusly, if you're using an older Mac and you're finding out that you can't get direct rendering to work (you can test this by running 'glxinfo | grep direct') then lower your bit-depth to 16 instead. You can do this by editing /etc/xorg/xorg.conf and changing "DefaultDepth" to 16 instead of 24.
2.) Adjusting DMA on your hard drive
By default DMA is turned off on most installations of linux. You can test the speed of your hard drive cache by running 'hdparm -T /dev/hda' and if you so desire you can try to tweak the speed a bit faster by enabling DMA 'hdparm -d1 /dev/hda'. This is a safe command to run while the system is running. After you've run the hdparm -d1 /dev/hda command, try rerunning hdparm -T /dev/hda. If you're satisfied with the performance boost you may consider adding it to /etc/rc.local
The first tip is the best, especially if you ever intend to play games on your sad little iBook of a system. I've managed to get quake 2 running at a reasonable rate-- something which I was afraid I'd never see. Now if I can get quake 3 running (something which worked rather well on Mac OS X, albeit, I had to disable sound), I'll be very impressed
If anyone has any other speed tweaks they'd like to share, feel free to post them here. Perhaps we can put them all together into a FAQ or HOWTO.