Slow Net on Wi-Fi, but full speed on wired, only on one device

I have two laptops, two Xboxes and one desktop.

I have a 30 mbps broadband plan with a turbo boost of 40 mbps.

I can easily download on a wired connection at full speed, but downloading on my laptop is only max 20mbps. The other laptop and the Xbox get the full 40 mbps speed.

And it's the overall speed, because I have an NFS share that I access on my desktop and if I'm transferring from it, my internet speed decreases, whereas total max speed is always 20 mbps on Wi-Fi for me.

I own a Dell N7010.
I have a Netgear WRN2000v2 running DD-WRT bridged with my TWC modem SBG6580.

EDIT:I have dell n7010 with a WiMax 6250 wireles controller.... my Config is ISP> Motorola SBG6580 > Netgear WNR2000v2 (DD-WRT) > Devices

PS: on ubuntu i manage to get full 3 MBps from net and while accessing NFS shares i get a good 6 MBps [when i'm ion same room as the AP]