Thing is, how many quality 300w power supplies are sold these days? Its likely a generic, or off brand unit that can't sustain rated power anyway. Keep a voltage utility handy when using this P/S and a new video card. Hopefully, if the voltage goes out of tolerance while gaming, the alarm will pull you out of the game before your power supply goes poof. I replaced a motherboard, CPU, and power supply in a customers emachines unit a couple years back, because his kid put in a video card it couldn't handle. When the 12v P/S rail went, it cooked half the stuff using the 12v rail. He had a 300w PSU as well.
Food for thought.