I have a mini-ITX board with VIA C3 CPU and CLE-266 chipset. The PCI slot holds a TP-LINK TL-WN651G wireless NIC which uses the Atheros chipset which is well regarded, supported by FreeBSD/pfSense and has given good service for over 16 months.
There are quite a few different brands of Wireless NIC that use a supported Atheros chipset. However be aware that some brands (Linksys, Netgear and DLink among them) change the chipset without changing the model number. For example, the DLink DWL-G630 might use a Marvell chipset (unsupported by FreeBSD/pfSense) or a TI Chipset (unsupported by FreeBSD/pfSense) or an Atheros chipset (supported by FreeBSD/pfSense) or a Ralink chipset (supported by FreeBSD/pfSense). One of the factors in my choice of a TP-LINK card was that they seem to change the model number when they change the chipset, making it easier to know what you are buying.
I have found http://linux-wless.passys.nl helpful in determining what chipset is used in particular products even though that site is focused on Linux. I know of no comparable site with a FreeBSD focus.