Just to confirm, you are using the 6870 as the display source, right?
The 6870 is getting awfully old, have you considered whether or not it is worth mining with at all?
Yes, it's a regular PC so far. If I can get it to work, then I'll venture onto something more exotic, multiple display adaptors, etc. I already have the hardware; so I thought I'd get it a try, but the driver nightmare seems unsurmountable..
Hmm, part of it is just finding the right driver/SDK combination.
The following pairs used to work well together, if you can find an archive with the old installers, I recommend trying whichever one will work.
Catalyst 12.4 2.7 SDK
Catalyst 13.1 2.8 SDK
Catalyst 13.11(beta) No SDK
Catalyst 13.4 No SDK
Cat. 13.6(beta) 2.8 SDK
Catalyst 13.9 2.7 SDK