Re: Minor, maybe major upgrades to old HP Pavilion a347x
dang man, she's runnin an ancient rig lol.
anyway, it looks like her hp's motherboard has little in the way of expansion slots, it only has 3 pci slots so if your 8600gt has a pci interface (not pci-e) then it would be compatible, if it doesn't have a pci interface then it will not work.
and it's kinda the same for your second question regarding the other motherboard, although it has more up-to-date slots (it as pci-e x16 slot, an x1 and 2 pci slots), if your 8600gt has a x16, x1, or pci interface it would be compatible with that second board, if it doesn't then it won't work.
just to make sure though, when you mentioned that second motherboard, i hope you weren't thinkin about swapping the hp's board with that second one because it wouldn't work, it has a completely different cpu socket and uses a different type of ram, again just a heads up if that's what you were thinking.