Right you def need an IDE (integrated drive electronics) hard drive.ATA UDMA it doesn't really matter to the average person.
There is no gaurentee that two devices will work in complete harmony.
Most PCs can drive 4 ide devices and more requires a RAID card.
Three ways to connect your IDE hard drive.
1.Run each drive on a seperate IDE channel if possible(recomended).Configure master and slave from the BIOS.
2.Use a dual cable, but connect the end connector to your drive you already and the middle connector to your new drive. Use the Manual jumper settings to configure the new drive to SLAVE.
3.As above but set jumper settings to both drives as CS (channel select). This means the PC will decide which one will be the master and slave (dodgy)