|Looking at your computer specs tells me you have extra SATA I connectors for additional hard drives. The issue before buying a hard drive is to verify the drive is SATA and it is backward compatible to SATA I speeds because your controllers are SATA I and virtually all new SATA drives are SATA II.|
Most are backward compatible using a jumpe on the back of the drive. Additionally, most controllers will just adjust to the drive. However you still must be cautious when buying a drive because SOME will ONLY work on SATA II controllers.
The installation instructions can be found by Googling for the phrase "how to add a SATA hard drive to my computer" without the quotes. Below is one example.
A couple of tips to follow. First one is to be sure to exercise static electricity precautions. You can do that the following way. Leave your computer plugged in but shut off the power switch on the back of the power supply in the case. If you have no switch (some don't) then you must unplug the computer. Before entering the case be sure to touch a metal part of the case to discharge any static in your body. Goes for handling the drive too.
If you live in the USA or Canada then I recommend buying the drive from newegg.com. Post the model of the drive you are considering for help with the compatibility thing I mentioned above.