I have a 240Mb/s Comcast connection. But my (old) router ports are 100Mb/s max until I get a gigabit router. That said, using the 5GHz radio on the Airport Express I am getting 94Mb/s on my Late '08 MBP. (The AE is in the same room as the MBP and there is a 100 ft ethernet line running from the (old) router to the office to the Airport Express.) 'Am looking forward to seeing the result when the gigabit router is installed (Monoprice have (an AC!) one for 39 bucks... .)
You will never get more than 100 Mbps on the Express as the Express maxes out at 100 Mbps on its Ethernet ports.