For PowerMac G4 with AGP slot or (possibly with minor mods, check Google) a Cube.
If you are running Panther or Jaguar with an old card (like the Rage 128 Pro) that does not support Quartz Extreme (http://www.apple.com/macosx/jaguar/quartzextreme.html), you are missing out. This upgrade is great bang for the buck.
$95 nVidia GeForce4 MX VIdeo 64 MB Card, ADC and DVI connectors
$70 ATI Radeon 32 MB Card, ADC and VGA connectors
Both are silent (no fans).
Located in Stittsville. PM or phone Gord at 831-1441
Price drop to $90 and $65