Type "Belkin G+MIMO long range tests" into Google (without Quotations) and read a few reviews. The theoretical maximum is 125Mbps and this varies according to range and local conditions. Any obstructions or conflicting wireless transmissions etc etc
If you are consistently getting 54Mbps, you`re doing better than me! My 11g router fluctuates all the time between 24 and 48Mbps which quite frankly doesn`t bother me too much. I`m waiting for the full N standard.