I haven't tested this extensively yet, but:

with my memory at 4400mhz, I get 29.2fps in Heaven at 1280x1024 maxed out with 8xAA

- with my memory at 5000mhz, I get 30.5fps at the same settings

- with my memory at 5200mhz, I get 31fps at the same settings

- with my memory at 5400mhz, I get 31.4fps at the same settings

- with my memory at 5600mhz, I get 31fps at the same settings

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Crysis:

5000mhz - 47.7fps
5200mhz - 48.2fps
5400mhz - 48.7fps
5600mhz - 49fps

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I will confirm this with more benchmarks when I can.

For now I am going to run at 5400mhz as it's faster and will probably keep the card a little cooler.

The memory generally needs to increase with the core speed. You cannot infer anything from one benchmark, btw.

The GTX 680 really needs a good memory overclock to take advantage of increasing core clocks and simply overclocking the memory gives a great performance boost independent of the core. So does the HD 7970, although is less affected by memory increases than the GTX is.

I updated the OP with more findings. It's pretty conclusive IMO. 5400mhz is the best, but I would probably do fine at 5200 or even 5000.

It's pretty conclusive for one benchmark

AND try two resolutions with heaven; also a higher one

What are the rest of your settings?

I'll get around to more testing when I have some more time.

I had all settings maxed out in Heaven. My CPU is at 4500mhz and the GPU core is at 1200mhz.

Others are mirroring my findings on overclockers.uk.

Apparently the memory has error correction that kicks in instead of giving you a BSOD and it slows you down.

Of course. You can find the memory limits easily. You are not at the limit as long as the performance continues to scale up with each increase. When the increase stops, you are at its limits


Also, when you have time, compare 4x vs 8x AA to see how increasing RAM speeds affects the framerates.

I updated the OP with Crysis benchmarks.

What GB/s do you get at stock SB?

My 470s improved with bandwidth all the way up to 182 GB/s, which is all the higher I can go.

Yeah, could be that 5600MHz is yielding a few errors, but you gotta make sure because testing with just Unigine could be within the margin of error for the difference of 0.5 fps.

Why not try 5800MHz and see if it increases yet again or not?

5800mhz is not stable.