You're very wrong in grasping power here. Wattage and Voltage are different, CPU does not give volts, the main power supply line does, 230V is from that line. Your PSU (power supply unit) determines how big of a GPU (graphics card) can you go for, and, for that, you need to check up how much wattage it is, that is where the requirement for 300W comes into place. If its anything above 250W, your system supports it, as the GPU itself only takes up 30W.