it sounds to me like you're interested solely in the tuning side of things. As such I'm not sure I would recommend putting a PC in your car.
CarPCs is what this forum is about. We have a section dedicated to OBD-II because it's a function that can be easily accessed via a CarPC. However, for you I would suggest using a laptop for this purpose.
Many times OBD-II has the ability to read factory fault codes and reset them. It's not usually a tuning program. For you to get something like this you need to access standalone tuning units. I think maybe reading up on what OBD-II can do first might be a priority: What is OBD-II?
by the way, for all of your OBD-II questions, I suggest you post in the appropriate OBD-II forum: http://www.mp3car.com/vbulletin/engine-management-obd-ii-engine-diagnostics-etc/
Your questions will probably be answered quit a bit quicker and with a lot more people with experience will see it there.
Might get something good out of that link too.