I'm doing the same for my STI. For the screen you can either check out the one from mechatronix or there is another from bybyte for cheaper. For the screen itself, the 2008 lilliput 629 seems to be the best bet since it fits both mounts and is cheap/bright.
About the motherboard/cpu, you won't have any stability issues with the embeded processsers. You just have to decide exactly what you want to do with the computer, then decide if that cpu can handle it. Most people run those for everything from DVD's to music to Nav and it runs fine. I'm a computer nut so I'm of course going to get a Core Duo pulled from a new laptop for like 30$. The core duo mini-itx boards are 200+$ right now though.
Lastly, start looking at front ends. Once you figure out what you want the computer to do, go make sure you can run everything on the front end you want. Most people choose RoadRunner, which is what I'm going with. The problem is, some software won't be skinable so it won't easily embed/run in roadrunner. I'm assuming this is the case of the tuning software you are running. If you check out the SUbaru skin one of the guys is working on, he found some software that I believe reads from a Tatrix cable and can monitor/log your car stats, but no tuning. That is what I plan to run so I don't have to spend 1k on Defi gauges. I'll just get my Utec protuned and leave that stuff to the pro's Good luck, just keep reading/searching. I have only been on here for a month or 2 just reading and have learned a lot.