First point you are getting your MHz and Mbps muddled but no worries
Max bandwidth for 802.11n is 600Mbps this is with 4 spacial streams using 64QAM and a guard interval of 400nS and 40MHz channels (channel bonding)
Thats the absolute maximum in the standard however I am not aware of anyone doing that at the presentr time.
Cisco maximum is 150Mbps on 2.4GHz and 300Mbps on 5.0GHz. This is because its at least impractical to channel bond 2.4GHz due to channel interference but you can on 5.0GHZ.
Each data rate is given an MCS (modulation coding scheme)index value.
The data rate can vary also by the guard interval which has two options of 800 or 400ns. If we take Cisco max of MCS 15 the vlues are
20MHz channel 800ns GI 130Mbps
20Mhz channel 400ns GI 144Mbps
40MHz channel 800ns GI 270Mbps
40Mhz channel 400ns GI 300Mbps
Smaller guard interval more bandwidth. Also if you double the channel width you get slightly more throughput than simply doubling the data rate as the bit where the two channels join is used more efficiently.
802.11n is not full duples its just alot more efficient than 802.11a/b/g its still a collision domain that uses csma/ca.