Just because you're using a Super-G 108Mbps adapter doesn't mean you're downloading at 108Mbps from the Internet. 108Mbps refers to LAN traffic, not Internet. Basically it means that if you have two machine, both with those adapters, they can communicate with each other at upto 108Mbps. In most cases your wireless LAN traffic will only be as fast as your slowest wireless machine, i.e. if you have two wireless machines, one with a 11Mbps card, the other with a 54Mbps card, all wireless traffic will be transferred at 11Mbps.
You're Internet download speed is dependant on the connection speed provided by you ISP.