^From what i know, it should be fine. If anything, your computer will bottleneck your GPU's potential performance. As long as it doesn't have different slotting version of pcie then your fine. However, the 6870 could be an overkill for an older system.
Sorry if that's innacurate, basing it on what ive known
Stumbling across this thread... i swear once this computer dies im going in to build another! Won't go too crazy hopefully.
I'm actually going to re-install my old 9800gt to see if its dead for sure.