STM, my man...you are trying to throw way too much money at this project!
I would use your 800W that you already have for the main PSU. That will run 2 GPUs at least. Then, get one of these for each additional GPU beyond the first 2. So, for example, if you would like to run 4 GPUs in the rig, then get 2 of these. That's only $50 over the price of the mobo and CPU.
Or, the other way is to do like F$ suggests and loop 2 big PSUs together. My question is why 4 in the one rig, though?

Are there any parts you can scavange from the 2 single core rigs - like HDDs or optical drives? They don't have to be high end, they just have to work. Then, when it's all said and done, I would sell whatever parts from those single core rigs are worth selling (like the 750W PSU, etc)...you don't need them around and may as well get some pocket change back...

...but, as you say - photography equipment you can actually make money with, so...I understand those kind of thoughts! Not to mention it's a blast.