You can't beat the Atheros support in FreeBSD. Why would you want to change the card? Or are you looking to move to 802.11n, which isn't supported in FreeBSD as yet?
The best wireless cards, IMO/IME, are the Atheros SuperG chipsets. You can tell which chipsets are SuperG chipsets quite easily: they advertise 108 Mbps throughput.
Pair these with a wireless access point/router that also does 108 Mbps, and you have a great setup. Fully supported by the ath(4) driver in FreeBSD 5+.