Depending on how many GPUs you want to fit, just give yourself the ability to use ~4GB RAM.
A nice AM3/AM3+ Motherboard paired with a Sempron will be fine and means you can support upto 4 GPUs on the go.
FM1 will only support 2 PCIe x16 lanes, so whilst it’s cheaper it has it’s drawbacks.
FM2 I’m honestly not too sure, but again, don’t go ‘too’ cheap and cheerful.
LGA2011 / LGA1155 / LGA1156 all support 4 GPUs but as usual, Intel products come at a premium.
It’s the RAM count that becomes more important, especially when using high(er) end GPUs.