any non-bus powered hub (read: that plugs into USB and wall) will serve you well. Go with a name brand one (I prefer D-Link) over less reliable brands such as belkin and targus.
If you have a HD that can be bus powered then all you need is any powered USB 2.0 hub. i.e., a/c adapter plugs into the wall. You must have a powered hub though. A non-powered hub will not have enough juice for two hard drives.
I don't know about external HD but based on experience and discussion with the Genius, only the usb port nearest the MagSafe can recharge the iPod Touch. So, if you connect the hub via this port, you might have a higher chance? Not sure.