Hi Derek,
I am in the exact same situation, just got the ugrade from Virgin this morning. The engineer left the Airport Express connected via an ethernet cable to the Superhub as it had been to the old modem. I found that without changing any settings at all on the Airport Express I was able to have my computer on the network from the Superhub, but still select my hi-fi speakers in iTunes (ie the ones connected over Airtunes via the Airport Express) and play music over the network.
At this point the Superhub and Airport were both creating wifi networks, so I tried connecting to each network in turn an found the Superhub gave me a slightly faster internet connection when testing the speed through a website. Using the Airport utility I then changed the settings on the Airport Express so that instead of creating a network it was set to "Disable wireless and connect to a computer or network via Ethernet" (that's on about the fourth or fifth page of the setup utility.) The AE rebooted with the new settings so my old network was gone, but I was still able to select my hi-fi speakers and play music over Air Tunes while the computer was on the Superhub network.
It looks like if you prefer it is also possible to turn off the wifi on the Superhub, connect the Airport Express to it via Ethernet and continue the use the AE as both your wireless router and for Airtunes, but my first impression s that I'm getting less audio drop outs with the way I've got it set up now than I did with the old modem-AE combination.
Sorry for the slightly long-winded reply, hope you get sorted.
Best,
Chris