computer psu has 5v too. you could power your 6v bulb off of 5v psu lead. it will be dimmer than usual but it should light enough. Or yes you can replace it with a 12v light and use 12v off of the psu. If you are going to replace the light anyway, consider LED's. They can be as bright with no heat and much less power consumption. And, for most practical purposes, they last forever.