Honestly you have failed to tell us what your budget is therefore how can one answer other than selecting out various cards & this leads to a UNEDUCATED GUESS/SUGGESTION! Can you afford 2? The crosshair v formula z motherboard is capable of SLI but WHAT POWER supply do you currently have? Guessing will often lead to assuming! Assuming is defined as ASS U ME ! NOT GOOD! PROVIDE more of your INTENTIONS, BUDGET, EXPECTATIONS!
Life is Hard, It's even Harder if your Stupid! John Wayne