Q: So there is no way it will work with that power supply....?
A: Power consumption of most cards is 50% or less in 2D mode then increases up to 100% in 3D mode. The new card that you want might be able to boot on your 300W power supply BUT as soon as you open a game, you will either get a blank screen or PC will just restart. So better just get a good 450-500W power supply. I suggest the high efficiency ones with 80+ marking.