by Steven J. Vaughan-Nichols - 802.11n uses channel bonding to increase its throughput. With this technique, your 802.11n device uses two separate non-overlapping channels at the same time to transmit data. Thus, you end up sending and receiving multiple data streams at the same time. Your 802.11n AP probably calls this using 'double-wide' channels. A 'double wide' takes up 40MHz of radio space instead of the usual 20MHz.
That's great ... when it works. The problem with channel bonding is that, in the United States, there's really only room for three 20MHz channels in the 2.4GHz radio spectrum assigned to Wi-Fi. If you use a double wide, that means you're taking up most of the space. Now that may be fine, if you're out in the woods where your neighbors aren't also using Wi-Fi. If you're in an office building or a town there's a good chance you'll be interfering with a neighbor's Wi-Fi signal and vice-versa with double-wide.
I'm not saying don't do it. I am saying it probably won't give you as much of a boost as you thought it would due to interference problems.
The way to avoid this slowdown is to, once more, spend some additional money for a dual band 802.11n equipment such as the Linksys Simultaneous Dual-N Band Wireless Router WRT610N, which is what I use about my house. By using the far less crowded 5 GHz band for channel bonding I can easily run HD movies from my downstairs media center to my upstairs HDTV.
To get the most from channel bonding and its wider Wi-Fi channels you need a dual-band AP that can handle simultaneous signals. Some older dual-band equipment, like the first models of Apple's AirPort Extreme could do 2.4GHz or 5GHz but not both at the same time. To maximize your performance you want to avoid this kind of hardware.
[ For more tips for getting really, really fast Wi-Fi, read Getting the most from 802.11n ]