Interference from cordless landline phones and microwaves can cause signals to cut out, but other than that, you probably won't notice a significant drop in speed by using the airport express instead of a direct hookup.
No consumer internet connection will go as fast as 54Mbps - that's the g maximum transfer rate for files shared between wireless g computers. Your cable internet should go no higher than 1 megabyte per second on average.
It's sad that some stores encourage users to buy 'super' g or 'MIMO' at 108 Mbps, making these people think their webpages will load faster. These speed quotes are only good for file transfer times over a network.