It could be that your power supply is not putting out enough power to keep up with demand. IE.. during high graphics loads. If you upgraded your graphics card without upgrading your power supply, this could be the problem. Most PC's off the shelf are under powered and putting a video card in that requires mass power will really kill a system. Or it may be overheating. Overheating causes a varied number of problems to a PC, most of them permanent.
Also, there is a code to the beeps. The pattern of the beeps will tell you what the error is. Check with the MB manufacturer to find out what they are.