just got radeon 6790 gcard .. with q8200 over clocked to 2.8Ghz with 4 gb ddr3 ram ... the question is Will my PSU 300watt run them ?? actually does the 6790 card pull a complete 500 watt !
It will just not turn on, and dont worry, it will not burn out or cause an explosion.
Even if it does turn on when you run any intensive applications, the computer will simply shut itself down.