The minimum requirements for the computer are not too heavy. There are some considerations from the standpoint of your applications for the Mixer in a Box that may influence your choice of computer hardware. The higher the channel count and the more complex the system the more power is required from the computer.
There are many people that are operating smaller systems from a laptop computer. For an ultra portable system with a lower channel count this is a very convenient way to go. Laptops are also used extensively as remote units. The remote functions can be set up over a wifi connection allowing the engineer to mix from any location in the venue.
For more robust systems a fast desktop computer is the better choice. These will allow higher channel counts with all the bells and whistles. If you are a production company I recommend a dedicated computer that is built and set up to run the SAC software.
The Sound Card
This isn't your grandma's sound card. For use with digital audio 'sound card' takes on a whole new meaning. The typical computer sound card has one or two stereo inputs and one or two stereo outputs depending on if there is a digital in/out available. There may be an output matrix for surround sound, but you are dealing with two discrete channels. Most live mixing situations call for many more than two inputs.
The sound cards used in audio work support many channels. They come in many shapes and sizes depending on your application. There are firewire devices and card buss devices for laptops and PCI and PCIe devices for desktop systems. Many of the solutions use optical cable couplings.
The production runs for these sound cards is limited. Because of this the sound card may be the most expensive single part of the system. The developer of the software recommends RME Audio as the source for sound cards, although there are several other solutions available. As long as the drivers are good for the devices there should be no problem.
Microphone outputs are low level signals. With any audio system there must be a mic preamp to bring the signal level up to a usable level. With a digital system the signal must then be converted to a digital format. Likewise, the output from a digital mixer is in a digital format and must be converted to analog to feed the output system.
There are many options as far as preamp/converters are concerned. One of the most widely used is the Berringer ADA-8000. While some of the studio types disparage the unit, for live applications it is very cost effective. These units provide eight inputs and eight outputs in a one rack space unit. They are among the most reasonably priced units on the market.
There are some optional pieces of hardware that operate with the system. The system is designed for remote control. Up to 28 remote computers can be set up to operate with the system over a network. Wireless operation of the remote is an option. It is possible to set up a system so that each musician controls his own monitors, or a monitor technician can use a remote to control the monitors while the FOH engineer takes care of the house mix.
Another optional piece of equipment is a control surface. This gives physical faders and knobs that can be used to control the system in real time. It is not difficult to do a mix without the control surface just using the keyboard and mouse, but occasionally there are situations where the faders can be handy.