Zune HD's are still available. 64Gb's seem to be thin on the ground though. Even two years on, this spanks any other non-iPod PMP IMO, especially if you pony up for the Zune Pass. It might also ease you into Windows Phone too...?
As for amp power, I guess it depends on whether you're one of those POS Grado-toting morons who uses them portably, and feels the burning need to share your music with everyone else around you. It drives anything up to my (600-ohm, nominal spl 102db - admittedly these are very efficient for the impedance given the super-magnets), Beyerdynamic Tesla T1's OK for home use at above sensible volumes and drive my (24ohm, 106db) Sony ZX700's to ear-bleeding levels, let alone anything smaller/more efficient.