You may hear an annoying hum from your speakers when running X. This occurs with some PCI and AGP cards which hog the PCI bandwidth to achieve better performance, and time-critical things such as audio get bumped.

Add the following into your XF86Config, under ServerFlags section:

    Option "pci_retry"

This works both under XFree 3.3.x and 4.x. This tip was tested under XFree 4.1.0.

The "pci_retry" option is pretty much undocumented.

A good explanation of the phenomenon can be found in http://www.zefiro.com/vgakills.txt.