802.11n can support both the 2.4 GHz range of b and g and the 5 GHz range used by 802.11a. Your wireless card needs to be dual-band for that, though.
802.11a is a bit rare to find as an active network. The 5 GHz range may be less crowded, but at the same time, the radio waves don't penetrate walls and other objects as easily.
Casey