First... I hope you removed the old power supply... Lol
anyway... It sounds like a problem with the video card and the mother board... Make sure that the video card is in its slot nice and tight (Do not be afraid to push it in a little bit... NOT WITH A HAMMER THOUGH! I LEARNT THAT THE HARD WAY!!!)... If it still doesnt work remove the videocard and get yourself a can of spray air... And spray inside the slot... Then put the card back in... If it still does not work call GEEK SQUAD
GEEK SQUAD RULES, GEEK SQUAD RULES!