I've found that using wireless has a negligible effect on latency, getting a ping as low as 15-20 on CS servers. Sure it might be a little lower if it was wired, but unless you have low signal strength then it should be so low it doesn't matter.
Internal cards and USB cards don't really differ too much from what I've found, just get a decent brand like Netgear, Belkin, Linksys and you should be golden.