Okay, I can confirm that by following the instructions in PJ's document, you can indeed stream over ethernet (though the instructions are a little out of date as the Airport Utility has been updated so let me know if you need to do it and I'll walk you through it).
However, I'm pretty sure you don't actually have to do this now. Having read through the document and played with the Airport Utility, as it shows in point 8 of PJ's document, there is a tick box labelled "Enable AirTunes over Ethernet" which should do exactly what you want (I never really paid any attention to it before since it didn't work for me, but it kinda makes sense now). Because I don't have an existing wireless network, I use the three Airport Express units dotted around my flat to do this. However, as I remember, I had a lot of trouble getting a network set up where you could walk throughout the flat and have the client switch from one access point to the other seamlessly and it looks like I solved this by setting all three AE units to "Participate in a WDS network" so there isn't one main unit actually creating the wireless network. I can only assume it's this which is preventing me streaming tunes over ethernet since, as soon as I switched back to my original settings, streaming over ethernet no longer worked once my Mac had connected back to the wireless network. All very strange!
Anyway, getting back to the point, it's worth going into your Airport Utility and clicking on the Music tab at the top and ensure the "Enable AirTunes over Ethernet" box is ticked. In theory, you should then just be able to connect your Mac up with the ethernet cable directly into your main wired network (not the Airport Express unit mind - your main router which the Airport Express should also be connected into). As PJ suggested, you could also use gigabit switches (which is what I use) or fibre, but if it's just the one Mac and likely nothing else upstairs, a single cable will do. With that, you should then be able to stream over ethernet.
Oh and should just add good work to PJ (the supposed AE novice!) for picking that up. No use to me of course (it would seem anyway), but definitely good to know it's possible!