I have a 2 year old Fujitso Siemens Scaleo P computer. I want to upgrade its graphic card from an Nvidia Geforce 8600gt to an ATI 5770 XFX version. However my computer only has a 400watt PSU and I want to upgrade it. How do i know which one to pick? All i know is that it should be aroung the 500watts but i know nothing about the ampers, volts and even the different cables... Does anyone know what i should pick?