Uhm nooo. Nowhere near 54Mbps, that's its maximum is it not? But your internet connection will be nowhere near as fast as that.
If possible plug your connection straight into your computer and compare the speed to the speed you get over the wireless connection, that will determine if the problem is your ISP or your router.
Correct. 54Mbps is the maximum possible speed, under real world conditions, you'll probably see something like half. However, seeing as most broadband connections are less than 10Mb, you should still be able to use your broadband connection to its fullest extent over wireless.
I would also agree that this seems like an interference issue. Also, if your wireless network is not secured, it's very likely that others are connected and using it.
I've also seen situations where on secured wireless networks, maximum bandwidth is not achieved unless you're using WPA or WPA2. See the Wireless Networking FAQ linked in my signature. That should get you started to ensuring your network is optimized.
I also would echo Starky's suggestion that you make a wired connection to your router and test to isolate the problem to wireless connectivity.