|The power supply unit may be having a hard time to cope with the recent hardware changes and may be overheating as you have also guessed. Overheating depends on many other factors as well, such as ambient temperature, title of the game, how fast you play the game, etc.|
You should calculate the power requirement of your girlfriend's PC and buy her a new PSU if the power need has increased dramatically.
You can find a good PSU calculator HERE and HERE
You may also consider re-installing the gfx card drivers, updating the other hardware drivers including the chipset drivers, etc. before making the decision to buy a new PSU.