fight might spin because the card won't produce video signal but there is power running through it so the fan gets powered.
your 4870 should still work without external power, at least it should go into windows in a sort of "limp mode". try testing the ram memory at another computer?
Thats weird then... But why no POST when it's plugged in?
And it doesn't, when the 4870 is in I get a graphics card error POST. And I know the RAM works, because it always has