you will see a bottleneck from the CPU (whether or not u overclock it a lot) and from the motherboard. your board functions in PCI-E x8 when in SLI. this will drastically reduce the performance increase you see with this combination.
Not to mention when you have SLI'ed cards there is what is called micro stuttering. However when the cards are operating at a slower bandwidth on a slow CPU you will see macro stuttering (visible to the human eye). Even if it increases FPS in games it may be very choppy.
Ask yourself if it is really necessary to go to SLI... maybe you could wait a while and buy a new board, CPU, and GPU.
It would be a bottleneck. There would be a performance increase, but it would be minimal, as in not worth the price/performance of the second card. If you plan on upgrading CPU and monitor in the near future it might be a good investment, otherwise no.
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