This really depends on the car (and the year it was made) and how popular is the computer (laptop) based tuning for that model is.
If you intend on getting an STi (or a WRX in general), there's a guy who writes nice software from South Africa. Google him out on the net.
You can help yourself with installing a carputer by reading through these forums, but it's better if you check a forum dedicated specifically to your future car's model to know what options are available.
Normally, you'd need to know if their tuner box is interfaced via Serial or USB, what the minimum requirements are, whether there's a real time mode, etc. Then you can choose your carputer accordingly.