It's not all about the watt consumption, but the amperage, you should aim for ~40A in the 12v rail. or ~20 in each 12v rail if you have two.
and with watts i wouldn't consider even trying to use a 300w PSU in any pc with a non-onboard GPU, at least not with the current consumptions the mobo, cpu and disk will need the entire PSU output when at max load. so, if you are planning on installing a gpu, get a nice 600w PSU with a high ampere load.