Which of these graphics card options will perform best with BOINC?

[NakedCOOL]

Hello guys.

I'm looking to change my old two 8800gts in SLI for new ones, and I'm considering several possibilities.

The economic option…

A) Set up a crossfire with three R7 250 since they only take up one slot.
B) Set up a crossfire with two R7 250X

At about 90-100€ each card.

These two, they don't shoot me in the back in terms of system consumption, nor do they cost a fortune.
Or, since we're already on this path...

The most powerful option...

C) Set up a Crossfire with R9 270
D) Set up an SLI with two GTX660

At about 150€ each card.

Which option do you think will give me more than the first option (the economic one) and the second (the powerful one)???

And which one would you choose???