It says that AP extreme works only if you have a FW800 port. If your machine looks like that, you probably don't have one. The first powermac to have one, I believe, was
this.
You'll probably have to hunt on
eBay for a regular airport card (not extreme)..
But first, make sure you can actually have an airport card. Click on the apple that opens a menu, then click about this mac. Click more info, then look for machine name. Mine says
Machine Name: Power Mac G4 (AGP graphics)
Yours should look similar, with different info possibly. Any machine older than mine does not support airport. If your machine is one of these models:
PowerMac G4 AGP, Cube, Gigabit Ethernet, Digital Audio, QuickSilver, QuickSilver 2002, Mirrored Drive Doors, you're in luck.
And yes, your processor's heat sink will not be purple unless you have an upgrade from sonnet like me. Or if you painted it purple, which I do not recommend.
I have a red video card as well, for the record, just in case that throws you off.