I don't have experience with servers but I'll help as much as I can. I think all SATA drives can hot-swap, I've checked on some of my desktops and it always worked. For NAS and such I don't think it will matter if you go Intel or AMD and both AMD and Intel motherboards have Gigabit Ethernet and most boards with both companies have well over four USB 2.0 and/or 3.0 ports.
I don't know much about unRAID is but I skimmed through the FAQ a little and it seems like a good solution.
I don't know your exact performance needs but I can assume a quad core is ideal here but I think it wouldn't matter too much whether or not it was an AMD Phenom II or an Intel i5/i7 quad core.