FYI, your GTX560s at stock speeds are faster than a single GTX580. As for your overclock settings, only you can determine that. All cards are not equal and what works for one doesn't mean it will work for another. You'll have to do some testing to see what your cards can do and at what voltages. Try incrementally raising your clock speeds until you get instability like driver crash or system hangs. At that point then try raising your voltage and re run the benchmark and see if there is still instability or if the issue subsides then proceed to raise clock speeds until it happens again. Repeat this process until you are satisfied or until temps begin to exceed 90C load or there is no appreciable gain in performance.

Wow, my score was around 7890 ish with the 850 mhz clock. Isn't it unsafe to clock at rates higher than 900?