If you only need the "N speeds" between your PC's and etc (but not to your modem)...
You could try...
a) modem connected wireless to old router in bridge mode (on wirelss b/g --> with an SSID of "modem" for example)
b) then connect your 825 to the back of the bridged wireless router (not in the WAN port, but in one of the normal ports)
c) Then connect your PC's and other devices on your 825 (wired and/or wireless)
--> Make sure your b/g wireless on your 825 has a differnt SSID and is on a different channel.
d) make sure that your 825 is not setup for DCHP, DNS, ect. (Your modem will have to continue to do this work.