Look for the features you want/need first. Do you need dvd? MP3? Surround sound? Most HU's will give 4v preouts and that's fine with most amps. However, some will give 6v - 8v. Those cost a lot more. Higher volts will give cleaner sound.
Like Isaac said it makes a cleaner signal to the amp if the voltage is higher. Some of the low - mid grade H.U.s have 2v preamp outputs as well.
One more thing, Make sure your amp can handle whatever voltage you are putting in it. For example, A certain MTX amp can only take up to 5 volts of input power. So an H.U. with a 6 Volt rating is too much. Im not sure if there is a way to lower the voltage or not on an HU. Maybe sum1 else knos