If you are using it as HTPC only, just get the lowest $$$/Gb drive out there. You play movies and stuff at approx. 1Mb/s (even for blu ray it's about 5Mb/s). The slowest HDD you can find probably can do 35-40 Mb/s. Fast HDD gets you nothing. And the Caviar Green saves power
As for partition/separate drive for OS, just use your old drives or create a tiny partition. Once again won't make any difference.