I was thinking about building a custom ECU computer for my car. My friend did it on his Civic and he cann control virtually any parameter from his laptop. He used a cheap Sempron, 1Gb of RAM and a 16Gb SSD. What do you guys think?
I believe he was able to get a 20-30HP increase his totally stock Civic just tuning everything up right.
I mean this in the sincerest way possible: you've got a lot of learning to do before you start messing with the ECU and melting your engine, I wouldn't recommend it. Stick with canned flash tune for now, that's good for 20-30 HP in most cars, mostly because the safe stock tune is needed to pass validation tests for engines which are ridiculous and far exceed what any engine will see in the real world.
But if you really insist, and just want to spend your money and sound cool, the Hydra Nemesis is probably the most popular true DIY ECU. Hope you know more about computers and electronics and soldering and signals than you do about cars

I guarantee you won't actually legitimately need it until you start approaching and exceeding 400-500 HP (ie: after you've racked up an honest $10,000 in engine work and can't go any further). Today's OEM ECUs are pretty flexible and really only lack map resolution.