You should NOT use a wifi card as a nic on a MD, nor a core, since PXE booting is required. You can use an hardware only access point, that talks to another hardware access point where there are configured as a bridge. If the access point are in the 2.4 Ghz range, then there will be lots of interference and your experience may not be great. MD losing connection to the core. Many linksys/netgear routers and the like have a bridge AP (access point) function that should work as well. Some of the newer draft N have multiple antennas and work in the 5 Ghz range so they might work better.
Note that adding wireless to your network can greatly compromise the security even with MAC filtering and WPA2.
If you do decide to go the wireless route, report back with your experience.