For my always-on machines I have a couple of Atom 525's with perhaps 30 TB of data storage. The OS for those is Scientific Linus 5.x (someday to be Centos 6.x).
These are plenty powerful enough for the services I use them for - files storage, light duty web serving, personal IMAP, DNS caching etc. and sip at the electrical supply.
They are good enough for light duty web browsing as well.
For more challenging applications (like games, photo editing etc) I have a couple of machines running 4 and 6 core I7s with 24GB of RAM. These only get turned on when I need them.