I would go for an SSD if you can. Much faster than a HDD and a much better experience overall. But if you need a HDD, anything by Samsung and Western Digital are great. I would get 7200 RPM, but a 5400 one is good enough too. Anyway, a HDD cannot even saturate SATA 1 (I believe), so SATA 2 or 3 is pointless. Get the most cache you can find, but 32 is good. Physical size depends on your computer, but most should be 2.5". If not, then it will be 3.5".