It's called cognitive dissonance... when you perform a behavior (in this case buying an expensive vidcard), and it goes against what you're hearing, it creates a 'mental conflict' (dissonance)... ie, people are saying your card is crap, but since you already did the behavior, it's difficult to change that, so inorder to remove dissonance, you change your mentality to really believe that your card really IS good (because you're behavior is done already, and it's a lot hard to change your behavior then your beliefs).
All of us have done something similar at one time or another.