Having configured a number of wireless networks before, and based on the tests you have already performed, I would say that its a security protocol problem, with a problem like this if you can change from wpa to wep, as others have suggested, I have found this to give noticable gains in transmission speeds.
( The problem being that alot of soho routers dont have the mem/cpu (balls) to deal with highspeed signaling (54Mbps) and lengthy encryption bits (128bit) )
This is based on :-
1) You know your line gives top performance with a direct connect usb type adsl modem, ruleing out adsl line associated problems..
2) You have tried the new router with a direct cabled ethernet connection.. and it gives similar results to test 1..
Try one speed test with wpa, wep and unsecured if you can.. that way you can decide on the best privacy policy vs performance...
Hope this helps..
5dayz..