I'm unsure about USB Hubs, though I was only thinking about this this morning. To get an external DVD drive to run, so I wouldn't need to worry about ruining the Wii's laser when running GC backups (as I read I could do this, but I'm still unsure if GameCube backups will read through an external DVD drive).
Additionally, my HDD that I use is a 320GB Verbatim, and is powered through both USB ports. As for getting USB Hardware to run alongside a HDD, you will need to install Hermes cIOS 222/223. Then it *should* run. For more information on Hermes, click here.
Hope that helps