This is my first time putting together a computer... I ordered the parts through newegg, and took a suggested build from the stickied thread here. Since the computer was for someone else, I let her choose her own case, which is the right size and everything, but it came with its own 500W power supply. The case wasn't expensive at all.
I'm wondering if I should just send the ordered power supply back, and trust the one in the case... or should I go with the 400W power supply that I ordered for the case? I'm wary, since the "extra" power supply I have cost more by itself than the entire case did.