|You provided no system specs. How do you expect to get help when we have no idea what you have?|
The HD 7950 is high end gaming card that requires a minimum of a 500W power supply. And considering the cost of the card ($300 +/-), the PSU should be a darn good one to protect your investment.
When you say "crashes", does that mean you're getting BSODs? If you are, post the info from the blue screen.