Check the power consumption of all your components, separating them into the various voltage rails (3.3, 5 and 12V) and compare these with the output of your PS. If you get to the limit by adding another internal HD, you may consider getting a somewhat more powerfull PS, like for instance a Thermaltake PurePower or SilentPower 680.
To establish power consumption, use the support pages of the various suppliers. You can also check Tom’s Hardware and do a search on powersupplies. They have some good guidelines on how to establish what your power requirements are.
Hope this helps.
Harm Millaard