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.
Well two of them could max about 800 mhash/s so do not think you will be getting 1 block every few days. It will take you about 20 days I think at this rate to get a block. Just something to think about before you think your gonna just make cash with this thing.