I understand that sometimes there may be a situation where the PSU is way more than required but in this case of mine, I'm not too sure.

Sometimes when I play games, my computer will crash and restarts itself, 10 mins into the game. Once I received a message that says something like the power is overheating or something like that.

Ok, so I have a 500W PSU. I have: 1x Internal DVD writer 1x SATA 250GB HD 1x Nvidia 8500 GT 2GB RAM.

As I'm planning to get an additional 250GB SATA HD, I wonder if I need to increase my PSU as well -- in full knowledge of the previous crashes experienced before.

Should I upgrade my PSU to 650W perhaps, or is that excessive?