In other threads people have reported that their factory installed MP wi-fi cards are connected to the wrong antennas. This results in terrible reception. You might want to open it up and double check the connections. I would do that first.
Failing that, OWC carries mac-compatible PCI cards and USB sticks.
http://eshop.macsales.com/shop/wireless/
They tend to be a little more expensive than most, but you get peace of mind and good support.