A couple of things that are overlooked on all upgrades are:
1) Airflow. As most cases have the drive bays at the front, where air is drawn in, make sure that you don't restrict the flow. Also, if adding additional drives, try and leave a space between them in the bay.
2) PSU. While adding a second drive is not always a massive overhead, there are some really ropey power supplies around, which don't necessarily do what they say on the label. This is an area which is always overlooked and it can be a costly mistake if they pop. IMHO, this is an area where you shouldn't make cut backs.