Graphic card problem computer won't start

Hello, Im not really that good with computers and i need help, a couple months ago my 6 year old HP comp would have these random shut downs while playing games. And then one morning my computer would not boot up, there was no signal from the monitor, keyboard and mouse would get no power, so i removed my 8600 GTS Geforce graphic card and allowed my comp to run on the onboard one and the computer was running fine...

i then turned to the power supply, my card needed minimum 300 watt i didnt meet the it so i went out and bought a 450 watt power supply, but the computer STILL wouldnt boot up with my card :( so i took it out again and ran it on the onboard one. what confuses me is... well a few days ago, my computer would not boot up with the onboard one, the monitor/keyboard/mouse getting no power. It only boots up 1/50 tries, but ive come to the point where no... it wont anymore

i tested my 8600 card in another computer, and it worked fine, i also did the eraser trick on the ram sticks, and tried to find a faulty one, no luck that wasnt the problem, i tried alot of those power up tricks such as hold the button down plug it in still no luck. I dont think its my power supply since both cards dont work and ive checked that its connected. Is it my motherboard? my processor? please help!