Re: Intel Atom Home Server Build, Help
The Atom will likely not work entirely for what you have in mind. You can get an Intel i3 2100 with a TDP of 65w (which yes, is a bit higher than an Atom, but so is the performance) and you'll get a several times improvement in system performance over an Atom. It's also worth noting that depending on the Atom, Dual Core is a bit of a misnomer if you're not careful about which one you're buying. Some of them will be "dual threaded" but only be one core with hyperthreading, and their performance for this task will absolutely suck. The Dual core Atoms are a little better, but not by much.
The problem is that the Atom is an in-order processor, and can't efficiently handle tasks outside of what its pipeline can do. Out of order CPUs like Intel's non-Atom CPUs and all of AMD's chips are much better at out of order operations. That's not the whole story mind you, but it helps explain the chip a bit.
The i3 is my current pick for low power computing because it's a dual core with hyperthreading and it's based on the newest chip architecture from Intel, which is just wicked fast.
The other part of your question is partially answered by Chipeater, and that's to check out those 80 Plus rated power supplies. It's also worth noting that it doesn't really matter which power supply you buy as long as you buy it for the system at hand. Overloading a power supply would be bad, so in this case, buying a 300 watt power supply will cause inefficiencies in the unit, and could even overload it if you get an i3 with the other items. But if you buy a 500 watt power supply, it likely won't be overloaded and will be more efficient. Basically what it boils down to is that it doesn't matter which power supply you pick - a 500 watt unit doesn't use more power from the wall than a 400 watt model does, unless you overload what the 400 watt model can do. There are folks I know with 800 watt units and they pull the same amount out of the wall that yet others do with a 650 watt unit. The wattage rating is for the components in your system, not what it pulls from the wall (again, unless you load the system up to the point where the PSU can't function properly)
Hope that makes a little more sense for you.