no you don't need to buy a drive you should be able to connect to the PATA on your motherboard with your old drive, ide ata, pata same thing, i only suggested the 500 gig drive for the extra warranty and space, but like you said you don't need it
should mention your drive is older, they do have limited life span, but that dosn't mean it will die in a month, might last another 4 years