Assuming a normal system, anything that can output around 250W would do fine. Most quality 380W PSU can do it.
Not really. Like I said you can never have to much power. I have a 850Watt PSU. I don't use that much power but if I want to upgrade hardware later I don't have to get a new power supply. I would rather have to much power then not have enough or just barely enough. 500 Watt's isn't that much more anyway its not like I'm telling him to get a 1000+ Watt power supply.
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.
Except it doesn't work that way. My PSU has two rails, each individually rated at 22A. According to what you wrote above, I should be fine. But the PSU can only provide 34A altogether, which falls short of your 40A mark.
The best thing to do is find out how much power you need, and then buy a PSU that can reliably provide that power and more. If you need 200W under load, don't buy a 650W PSU. If you'll be CF/SLIing, make sure you have enough for your cards and the right connectors.