Wireless is very generalised and mis understood.
You need to examine your Wireless configurations before you can try and adjust speed.
Many new routers come pre configured to run in mixed mode, using both the
n standards. This is great if you want all your devices to work together but it becomes a real problem when you start to transfer large amounts of data.
You can mostly solve speed issues by changing your router into a dedicated mode. Either
n. The reason is that ALL clients connect run on the same channel and frequency. The router does not have to switch between modes. The problem becomes apparent on heavily used routes in mixed mode where it seems to stutter, loads, stops, loads, stops, loads, stops.
To overcome speed issues I have configured my newest router to use Wireless N 5Ghz and enabled dual channel "40 Mhz" giving me 70 Mbps~150 Mbps per channel. Another older router with Wireless G 2.4Ghz. Both are dedicated to those modes and if I cannot connect to the newest N standard I fall back to G.
That ended all my internetting Wireless connectivity issues and ensures the Routers handle data at the maximum speed.
My Pi uses a Wifi n USB that is capable of connecting to 5Ghz network
and I can send files to it at a speed of 50Mbps (about 6.25 Megabytes
per second) - That is the maximum limit also caused by a cheaper SD
card I am using but I can read from the Pi allot faster!
You should check that you are not cross talking with other routers on the same channel. Use inSSIDer to evaluate what channel you are on and which ones are free to use. This can dramatically increase Wifi performance too!
FYI - I suggest you do not overclock your Pi. This causes problems with Wifi and LAN. This is one of the first things I also enabled and it really drove me mad with the poor performance of Wifi.