I want to build a server with the cheapest parts available (preferably under $300 USD) and also be energy efficient. This server will only be doing file transfers (usually Word documents, possibly pictures?) and may be on 24/7. I plan on using either FreeNAS or Windows XP Professional's built-in FTP software.
I've looked into Intel Atom and AMD Sempron. Currently Newegg is selling a very cheap Lenovo nettop that includes an Atom D410. I can also build an AMD Sempron 140 system around the same price. Does anyone have experience with either of these CPUs?
Since it doesn't seem you have a lot of heavy requirements a cheap single core nettop should work for you. They are very power efficient too for always on. If you run out of internal hard drive space you can either swap out the internal for a bigger one or add an external usb hard drive.
That deal comes with a 1TB HDD so you should be good to go. The best parts about that deal is it's server-class parts (Supermicro is very good - read the reviews), low power (80+ PSU and dual-core atom) and the mobo has 6 SATA ports while the chassis also has room for 6 3.5" HDDs (4 in 3.5" bays, 2 in a 5.25" bay with an adapter).
Even if you go smaller you're not going to get cheaper.
Given those requirements, you can go even cheaper believe it or not. I however cannot vouch for the quality of the barebone though it gets good ratings. YMMV. I think there's even a 2nd 3.5" bay in the system should you need more disk in the future.