re: hard drive interface cable

I just bought and installed a second hard drive on my 4300 dimension dell computer. The new hard drive is a Seagate ultra ATA/100  120gb. The installation seems to have gone well-althaugh i don't consider myself a technical expert- and my computer recognizes the new drive once i formatted it with the utility disc which seagate has provided me. Now I have few questions for you experts. Why my computer recognizes my new drive as a 111gb when it really is a 120gb. Second the manual that came with the drive recommends that  i change the old interface cable with the newone which they have provided me.Is this necessary? And why? Third, how in the world you disconnect the old cable from the motherboard? I tried and is very hard. Beside I dont want to bend or break anything. I tried to pull straight up but without success. Any advices? Thank you. Frank.