After being a little ticked off that I had to set my AEBS to b/g in order to access the internet and stream music at the same time with an Express added to the network...I found this tip courtesy of Duane from

You can keep "n" only on your Extreme Base station, connect to the internet, and stream music via Airtunes at the same time without switching networks or joining a network with your Express.

* Set up the Airport Extreme Base station as a Dual Band Network:

1. Connect the Express to the AEBS via ethernet cord
2. Create a new network with the Express and use the following settings:

Under the "internet" tab in Airport Utility:

1. Connect using ethernet
2. Configure using DHCP (it will list an IP address, subnet mask, router address)
3. DNS server
4. Domain name = ISP
5. DHCP client ID = blank
6. Ethernet WAN port = automatic/default
7. Connection Sharing = Off/bridge mode

Under the Music tab make sure you have Airtunes enabled

Now you can set your AEBS to "n" only at 5Ghz with a multi-cast rate of 24Mbps, connect to the internet, and stream music at the same time.