well, with the old at/xt cases, all you would need would be a case with a power supply in it.
then you could just turn it on, and the fans could run off the power leads.
the newer atx cases, and power supplies, require a signal, you can find this by doing a google search for "atx pinouts" (if it's typical, grounding pin 14, green should light it up)
so either way, you just need a case, and a power supply.
but with the atx, it's a little more difficult to make it start up.
then you could just turn it on, and the fans could run off the power leads.
the newer atx cases, and power supplies, require a signal, you can find this by doing a google search for "atx pinouts" (if it's typical, grounding pin 14, green should light it up)
so either way, you just need a case, and a power supply.
but with the atx, it's a little more difficult to make it start up.