Don't get a plug-in chip. They will leave a trace in your computer. Get a plug-n-play like the superchips microtuner, It also has diagnostics mode on it that can save you alot of money in the long run. You wouldn't imagine how much they can charge to run a diagnostics test on your car if you have problems.
The superchips micrtuner is very expensive though compared to a plug in chip. However, if I had to recommend a company, Superchip would be it. Esepcailly for this guy since he has an automatic and all and Jerry Wroblewski does work for Superchip. For those of you who dont know, Jerry Wroblewski was is an ex-Ford engineer who specializes in the 4r70w automatic transmission found in the 99-04 GT's.