Problems with new Virgin 50Mb broadband wireless connection.

I just got the Virgin 50Mb broadband service installed and it's not working properly. If I connect straight to the modem (via ethernet) I get between 20-35Mb max and if I connect wirelessly I get less than 1Mb! I have the netgear WNR2000 supplied by Virgin. I had 20Mb before this and it worked perfectly, never had a problem with it. My brother is using a PC and gets a constant 54Mb connection from 2 floors away!
Are there some settings I need to change?
p.s. I have enabled the 'n' capability already and just another interesting point - if I have the ethernet connected and leave airport on I get between 45-49Mb!?!?
Thanks in advance for your help

My mac is a MacBook (Black) 2.0 GHz Core 2 Duo Mac OS X (10.4.11)

Sounds like some kind of interference between you and the router. Any microwave ovens, cordless phones, baby monitors or other RF-emitting devices between you and the router physically?

The router is literally a foot away, if even that...that's why i'm so confused. There's no other electrical equipment around apart from a tv...