I was contemplating a 4870 or two, but then I came across quite a few people discussing the 4870 EDID problems. From what I understand the monitors don't have a signal under DVI - you have to use analog when it happens.
Can anyone explain to me when/why this happens?
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It's a driver issue. I had the 4870 with 8.7 drivers and the same problem. 8.8 fixes it.
It's not necessarily a driver issue. I tried to get a 4870 working and got no DVI output even when trying to see the BIOS before any drivers were loaded.