I've been playing around with desktop pc builders and from what I've gathered, my upcoming build will only consume about 210 watts of power. Among the power supply units listed in the shops in my city I've found models providing as low as 200, 250, 300 and 350 watts. As far as I remember, the golden rule is to always have a spare 20% headroom from the target power consumption, so I would probably be fine with a 250w PSU. But in case I decide to add some extra ram sticks and hard drives, let's make that 300w.
Until now I've mostly dealt with much older (6+ years) desktops, and the power draw there often required PSUs with >500w rating. And seeing numbers so low I'm not entirely comfortable, even though there is probably no mistake in the total wattage calculations. Is there something I should know about these modern low consumption builds? Will it really be okay if I get a 250w PSU for a system which draws 210w or should I go higher to 300/400w instead?