GTX550TI 300watt issue or something else?

I have a P71155 desktop and was trying to figure out why my screen goes black and no signal found after installing the driver but the computer stays on and the fan for the card is still spinning. I do know the psu for the card is recommended for 450w and I only have 300w so maybe that is the problem? or at least I'm hoping that's the problem and not something else. I have ordered a 550w psu that should be coming in next week but was trying to see if anyone knew of any other problems they could think of? my system specs are these. let me know if I'm missing something plz motherboard:Carmel2 processor: i5 2400 memory:6gb ram psu:300w