Great.
Usually dual/triple speed cards/routers auto detect the best speed that they can run at.
The old ethernet standard of 10/100, now being replaced by 10/100/1000, will also select the best speed automatically, provided that you hadnt selected b, which is what I assume you had done on either your router or your card.
As long as you select g or g and b then it will do 54mbps, as you have now found out