The Dell computer layout has room for only 2 HDD's inside.
It does list 4 SATA ports on the board however.
The CD/DVD is connected by an IDE cable.
One other consideration with the limited space is to purchase an USB to SATA external enclosure, place the HDD in that, and connect it by USB port on the back. Here is just an example of an external enclosure if that might help. Cost ~$28.http://www.amazon.com/Vantec-NexStar-NST-360SU-BK-External-Enclosure/dp/B000EDKO04/ref=sr_1_13?s=pc&ie=UTF8&qid=1350513651&sr=1-13&keywords=vantec+usb+2.0+hdd+enclosure
With your computer, you would need a USB 2.0 version, and I'm assumming the Seagate HDD is a 3 1/2" size.