802.11a, the maximum data rate is up to 54Mbps
802.11b, the maximum data rate is up to 11Mbps
802.11g, the maximum data rate is up to 54Mbps
802.11n, the maximum data rate is up to 150Mbps
54 Mbps is the maximum transfer speed of 802.11
40 Mbps or less
54 Mbps
Yes, almost every n-router available supports b and g-modes too.