I added a second HD to my system awhile ago. A few important points to remember:
1. Do you have a place to put it?
2. Is the IDE (I'm assuming no SCSI drives here) cable long enough?
3. Does it use a ATA interface that is higher than your motherboard supports?
In relation to Question #3, my mobo only supports UDMA/ATA-33. My second HD was one that supported ATA-66. So, I added a PCI controller card; in this case, the Promise Ultra ATA 66.
If you do that, make sure there are only drives that support that type of interface (ex: ATA-66 drives on an ATA-66 controller card).
Heat should be the least of your concerns. Good luck!
Amish
Be careful among the English, you hear?