Good enough for what?
I assume you are asking if a 300 watt power supply is sufficient to operate some sort of computer. The short answer is it depends on the individual components and what the pc will be used for. Different components have different power requirements. For example, a lot of the off the shelf slimline pc's with low power requirements come with a slimline 220 watt power supply that will run the system. It's okay for general purpose use like surfing the web and email. On the other hand an overclocked Intel Core i7 system with two high end video cards operating in dual mode that is used for hardcore gaming requires more power and sufficent current (amps) on the 12 volt rail(s). A high quality 700 to 750 watt power supply would be required.