so you decided the memory of 500gb. Then u must decide the clock speed and 7200 rpm is good for normal use and some games. Typically, the faster the disks spin, the faster the data is read and written.
then interface. PATA and SATA interface hdds. you should match a PATA drive to a PATA interface whenever possible, to get the best performance from it. SATA drives work with the SATA interface only. Inexpensive PCI Express and PCI add-in cards are available that let you add a SATA interface to a computer that lacks it; PATA and PATA/SATA models are available only for PCI.
Then decide about the noise levels if your usage is in room.