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 discussions.apple.com:

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.