I'd definitely go with the AirPort Extreme card. It's GUARANTEED to work with OS X (if it's new, that is), and it won't take up a PCI slot you might want to use for some other purpose.
ALL IEEE802.11g standard devices will work together. AirPort Extreme is just Apple's brand name for their 802.11g equipment. Further, they're backward compatible with the slower 802.11b, 11Mbps equipment.
And the WRT54G has a good reputation for working fine, both with Apple and non-Apple client cards.
Me, I'd most certainly go with the AirPort Extreme card if it's new - like the ones you can get from the AppleStore for about $50... Great price!