0.25 ohm, 1 ohm or 4 ohm
How you load an amp determines the amount of power out.
There is a trade off though, the lower the impedance (ohms) the more power is pushed as well as higher distortion and more heat is generated. The excessive heat will shorten the life of the amp.
For best overall performance, it's far better to get a higher powered amp and load it with 4 ohms than a lower powered amp and force it to push more power with a lower impedance (2 or less ohms) load.
More info here http://spkrbox1.spaces.live.com
