One of the nice things about Linux is that you can give use to your old computers. This weekend I put CrunchBang Linux (uses very low resources) on my old HP 6736 for mining with USB Erupters (not using any GPUs). It has a 667MHz Celeron, 256MB RAM, and USB 1.0 (http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph06109&tmp_task=prodinfoCategory&cc=us&dlc=en&lc=en&product=60421). I'm pretty sure I couldn't have done that on Windows, and the cost was free. To save watts, I disconnected power to the CD and floppy drives and the setup uses only 42 watts when running, and would be even less if I had used a USB flash drive instead of the HD to host the OS. (For really low watts, you could go with a Raspberry Pi).

With everything running, including cgminer 3.1.1, I don't go over 135MB RAM and CPU use is negligible. The accepted shares, HW errors, and other measurements are in line with what I see in my other setups, so the age of the computer isn't affecting anything.

So, if you have Erupters and an old computer laying around, and you don't want to buy a RPi, put the old computer to use. Your only cost would be a powered hub (if using multiple Erupters).