My x1600 runs nice full QE/CI and dual screens. The Jas 10.4.8 disk i have has "10.4.8.ATI.Radeonx1x00.agp.support.pkg" and there is one for pcie so i did not have to do anymore than tick a box . There is no slow down when using Exposé and get all the ripple from the widget.

My full specs are
Amd 3000 @ 2.2 ghz
Ati x1600 512mb
1gb ddr @ 333
nforce4 m/b Asrock k9updage AC97 Built-in 1 Ethernet
3 80g ide hard drives
2 ide Dvd-rw
Realtek pci 54g wireless card (runs great on mac shit on windows)
Belkin 3 usb2 and 3 firewire card
Thermaltake Case
All for under £300