Computer is crashing after video card changed?

I recently replaced my video card with this new one (ASUS ENGTX550 Ti DC/DI/1GD5), and whenever i try run any game, it crashes after about a minute of playtime. My monitor will just say no signal and go off. And the sound is like frozen when this happens. I can only force switch off my computer and turn it back on.. Please help? possible compatibility issues concerning power-supply or motherboards????|||I would look to the power supply. That is a big honkin' video card that requires a minimum 400w power supply.