Just because you have a wireless G speed of 54Mbps doesnt mean you will ever get anything close to that. You would be lucky to find somehting at 1Mbps.. Those speeds 54-108Mbps are used to make your interlan transfer of files faster.
It depends on the source you are downloading from. If you have wireless G UP TO 54Mbps... and an ISP connection up to 6Mbps then 6Mbps is your limit... but nobody uploads that fast, as I said before.