5ghz is good, by the way, I like it.
DSL modem is connected to Netcomm 802.11b/g/n wireless router. This supplies the 802.11b/g/ 2.4ghz N signal for iPhone, Xbox360, and anything else.
Then, I connect via ethernet from Netcomm wireless router to my existing Airport Express. Airport Express supplies a pure 802.11 5ghz N signal for my Mac, iPad2, new iPad and AppleTV.
This way, the Mac, iPads and AppleTV always "see" only the Airport Express network, hence WiFi iTunes syncing as well as AirPlay streaming is great, and the 5ghz network with maximum N speed doesn't suffer interference and/or dropouts such as when my microwave is running.
As for the iPhone, I'll just use a cable when I need to, but this way my iPhone and Xbox360 can access the Internet as needed without degrading the connectivity of the Mac, iPads and AppleTV. I don't play online games, I only need connectivity to update or download stuff on Xbox360.
Yes, the disadvantage is double-NAT, but I haven't encountered issues yet.