BTW, the only computer modification I know of that is reputable comes from JET Performance. It is a true reprogramming of your factory ECU and is customizable based on your other modifications. It costs $400, so i would only do it if you have other mods. The gains seen without other mods probably wouldn't be worth the cost, but with mods it would probably have a noticeable, though not huge, impact on performance. Large gains are only seen with turbocharged engines. keep in mind though that this reprogramming would likely also require the use of premium fuel, since part of the reprogramming is more spark advance. I currently have a K&N intake and Gibson exhaust and have the JET program on my list, but probably not until I at least get headers and the larger throttle body.
http://www.jetchip.com/