Three things could happen.
1. The video card does not perform properly. It may need 300W at full power, but may use less at idle. May lock up the computer when you try to run GPU-intensive applications.
2. The PSU draws power away from other critical components to try and power the video card, damaging parts or causing erratic system behavior.
3. The system overdraws power from the PSU, and burns it out.
Time to upgrade!
