I want to run two 5870s for bitcoin mining (since 5970s are literally impossible to find).
The question I have is this: do I need to crossfire in order to take full advantage of them? I know for sure that my board (an M4A87TD EVO from Asus with Athlon II X4 CPU) will bottleneck because it only has one PCIe 2.0x16, the other is 2.0x4. If I have to I'll shell out for a new board, but I'd like to keep costs down until I start pulling in some cash with this machine.
Best to xfire or youll have to use dummy plugs, doesnt use much bandwidth though so x4 slot is fine (people are using the x1 slots fine too

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Crossfire really doesn't make a difference, its different for every card/situation/setup. Usually your better out without it (requires a dummy plug if on Windows) (no dummy plug on Linux).
I prefer without.