I am currently in the process of setting up a SETI@Home cluster running on Raspberry PI boxes. Currently i am working with three nodes, and i will expand it if is a success.
For OS, i use Raspian (Debian clone), combined with some cheap 8GB SDCards from a random cheap vendor.
In order to install the Raspian ISO image to a SD card, i use the application win32 disk imager, which simply takes a ISO image, and writes it to a designaded SD card.
It is worth noticing that this will cause the partition(s) on the SD card to not be any larger than the image itself. But you can fix this upon your first Raspberry PI boot, as the application “raspi-config” will be auto-started. (if not, start it)