You could use any (approved) modem that has a capacity to handle the speed that your ISP is able to deliver. Anything above that is just a reserve for future incoming Internet speed updates.
If, for example, your ISP offers broadband at up to 60 Mbps, that would be the maximum possible Internet speed in your system. The AirPort Extreme base station can operate at much high speeds per se, but you are not going to get anything above 60 Mbps anywhere when connected to the Internet in this example.
As I mentioned earlier, the speed in a Wi-Fi network will be reduced depending upon distance and interference, so it is an advantage to have a base station capable of handling higher speeds. However, this does not mean that you can get more than the 60 Mbps at the computer, but it would at least ensure that the actual speed is as close as possible to this value. If you had an older 802.11g router instead, even a computer with fast Wi-Fi would be limited to receiving perhaps something like 15 Mbps only a few metres away.
There is probably a speed test service in your area (similar to www.bredbandskollen.se here in Sweden) that would allow you to verify the actual Internet speed at the modem. Remember that the speed can differ substantially during a 24h period.
Purely local connections (on your internal wired or wireless network) can operate at higher speeds.
Jan
