Hello Patrick,
The best way to set up a AP to get 300mbps is to use 5.0ghz on 40mhz band, not many wireless adapters support 5.0ghz so you might have to stick to 2.4ghz but it could cause a lot of wireless interfernce if you use 40mhz on 2.4ghz band. When you open inSSIDer while this is set up you'll see what I am talking about. The wave length will pretty much double.
So what configurations have you done though?
Thanks,
Clayton Sill