It could either be a faulty card, miss-fitted card or broken RAM.
Try removing and re-slotting the graphics card as with most customers that have tried to install a graphics card themselves, this has been the problem.
As for ram, slot in your old graphics card and see if that's working, when ram goes, it goes at the weirdest of times without any evident problems beforehand.
If the above suggestions haven't solved it, RMA the card.