It would probably be easier to use the access point for the wireless network since you already have it. Can you just set it to gateway mode? I like an access point in gateway mode hanging off a firewall like e-smith. Just secure that NIC. That way you can require IPSEC, or only allow pptp,ssh, etc on that card.
If you decide to go the card route, most of the cards are about the same. It might have a Linksys/Belkin/D-Link label, but it will be a Prism2/Lucent/Orinco etc, chipset.. If you are interested in an external antenna, check for that feature and don't worry too much about the brand.
Check out some reviews:
http://www.practicallynetworked.com/list.asp?p=5