You don't have to put it in the same place as the other HDD, any 5.25in bay will do, it would depend on the case somewhat, some (like mine) have 2 free 5.25in drive bays under the CD/DVD drives.
There would normally be 2 IDE sockets on a MOB, ide 0 and 1. So there may well be a spare connector on your MOB, have a look.