First make sure if you're buying IDE or SATA. Some older motherboards do not have SATA connections. If buying IDE, you can either connect the new HD to the same IDE ribbon cable, just make sure the new drive is set to slave. If connecting to the second IDE channel (IDE 1), make sure you have the appropriate IDE ribbon cable (80 conductor UDMA).
If you have SATA connections, buy the appropriate drive and connect. You might have to activate the SATA port in BIOS.
You will be asked to format the new drive. Just make sure you format the correct drive instead of your old Samsung.