Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If your Mac is Dual Processor, not Dual Core it must fit. I've checked in 2.3 Dual which is standing near me at this moment (i couldn't believe that might be any troubles with regular PCI card like linked) 😉
 
You got PCI Express G5, i.e. Dual Core not Dual Processor (w. AGP bus).
PCI card won't fit certainly 😉
I don't know any PCI Express wifi card supported by Mac OS. Best way will be USB WiFi dongle. Something like this or similar.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.