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Real world we're talking about 30% faster than the 5770. But you and I both know the 4830 OC's really far on the core. If they are overclocked we're talking 50% faster. I don't think it's worth it for DX11.
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Overclocking reasons can not be one sided.
If so, you could say that perhaps the single 5770 could be overclocked closer to stock clocked 4830 X2 speeds.
Overclocking in most cases simply cancels out.
Overclock the 4830 and compare to the overclocked the 5770.
Also, brands make a difference.
My Sapphire 4830 would not overclock much at all.
Just got the 5770, and have not needed to overclock it yet.
Other things to consider.
Can you sell the 4830's now, for a good amount still?
If you wait, how much will the value drop?
If I could sell, I would upgrade. No buyers, then no upgrade.
The 5770 will be more future proof and could later be paired with another card.