my friend of mine said if I wanted to, I should mod my audi A4. Increasing the horsepower to 250 and he said its done with a flashcard. Will this ruin my warranty?

wrong forum, and the software upgrade you are looking for is called a "chip." it is no longer a physical computer chip, it is a software download that gets put into your car.

www.goapr.com is just one of the tuners' websites that will do ecu tuning to your car.

as far as your warranty, it only voids things if your modification to the car, caused the problem, when and if a problem arises

takes apx. 10-15 mins.
Bottom line, it increase boost, timing and your fuel curve giving you additional Hp and torque.
I have used APR and Revo. Personally, I prefer Revo. GIAC and others offer these performance "chips"
You can also get a test chip and have it run for a specific amount of time to see if you like it.