Welcome to the Apple Community.
Hos Can i stream my homevideos from my nas to my Apple tv 3, without turning on a computer?
A nettop is what you are really looking for. And that will be one of the atom based options. It wasn't that long ago when people were getting Leopard (10.5) up and running on a netbook, or nettop. For the most part, you are looking for a machine that can have it's PCIe wirless card swapped out for a native Broadcom card that's recognized as an "Airport" card under Leopard. Brands that allowed this were Samsung, MSI, and others.
MSI might be your best option here, or any other computer, like an older Mac Mini running a PowerPC chip. Leopard (10.5) did run on both Intel and PowerPC systems. So you can Google and hack it onto a MSI nettop (with a wireless card swap), or get an older Mac Mini running PowerPC. Load up iTunes and go.
And both of those options will handle external drives. Have a great weekend. Who know, Apple could announce something else under IOS 6 on June 11, 2012.