ide cable but you can get a 40pin IDE cable with a 80 wire.
Then there is SCSI and Seriel ATA but I wouldn't go there with these 2.
We could hel you more if you give us more detail.
Are you upgrading ? or swapping a HDD over ?
Thing is with older machines if you have a old machine with a dated bios usually older than 1998 its unlikly to be able to recognise a HDD bigger than say 20GB and will only see it as say 8.5GB or near.