Alright. Normally I'm pretty well versed in this stuff, but I'm stumped.
I recently purchased a Sapphire HD 6870 graphics card, and after installing the card I've been able to produce two outcomes.
The first is the computer failing to boot at all. This occurs if I plug any of the PCI cables into the card. Doesn't matter if I plug both in, or just one, the computer fails to boot.
The second is the computer booting, but the graphics card's fan will run at full tilt (loud as hell), and doesn't put out any signal. This happens when I don't plug any of the PCI cables into the card at all.
the second issue is normal for a card missing full power.the fan trying to keep the gpu cool. for the first problem sound like the power supply can suppply the output needed for the card or you have a bad card. have you tried the card in another pc. and do you have another card to try in the video pci slot to see that the slot working.
what your old card...the 6870 needs 173 watts or 15 amps to run at idle and 25amp at full load. i would look to see if you can return the card and find one of the newer 7000 cards that used less power.
see if a friend has a spare all you need to do is hook up the pci power to the video card then green wire it. if it a power issue then with another ps powering the video card if the card is fine it should post.