Right, I'm just going to do some investigations into the effect of memory overclocking on my hd5870, the core clock on the card will be set to 900MHz for all the following results, i'll update this post as i do more benchmarks.
First up is left 4 dead.
Time demo was recorded for the first level of no mercy, from the rooftop start point to entering the safe room door, framerates recorded with fraps. Any aspect of human error has been included with results. settings: 1680x1050, maximum, 8xmsaa and no vsync.
900/1200: 118fps average +/- 2
900/1250: 117fps average +/- 2
900/1300: 118fps average +/- 2
So for the first test we see no gain from memory overclocking outside the bounds of any human error that could occur. From an 8% increase, we would at least expect to see a small increase in the region of 3-4 fps in a memory bottlenecked system. It is worthwhile to note that gpu usage was around 60% at most, while cpu usage was 50% at most, so neither were pushed to the limit, therefore the l4d test should be taken with a pinch of salt.
UNIGINE DX11 BENCHMARK
moving on to one of the more extreme gpu tests for the hd5870, this benchmark pushed the gpu usage up to 100% as expected, so the memory bandwidth should become more of a factor than in the l4d test. Settings used were 1680x1050, 4xMSAA, all settings highest and tessellation on. (Vsync turned off, ofc).
900/1200: FPS 31.9 [score 803]
900/1250: FPS 32.3 [score 814]
900/1300: FPS 32.8 [score 825]
this time around we see a linear increase when increasing the memory speed. However, the increase is only in the region of around 1% in each case. If the card were truly bottlenecked, we would expect to see a more drastic increase, but as it stands it does appear that the speed of the card seems to be pretty well matched against the memory speed. Of course, that may all change as ati improve the drivers to unleash more power from the card. Comparing the card to other generations is not feasible either, if the hd4890 was bottlenecked by it's memory bus, how can the hd5870 be so much faster if it is using the same memory? the increase in bandwidth is only around 33% but the increase in overall speed is around 60-80%. The numbers just don't add up, also remember that ati aren't idiots, if they had an epic card that would be severely bottlenecked by a 256 bit bus, they wouldn't used one. The card would cost more, that's for sure, but it would also be a helluva lot faster if it was severely bottlenecked. My experimenting with memory clocks has not yielded anything to indicated such a bottleneck thus far (and i have done others as well as the ones shown here), but as stated earlier, driver improvements in the future will likely prove whether or not the bottleneck exists.
However it could be entirely likely that the memory speed has a far lesser effect at the resolutions i am testing at, especially in l4d. If that is the case, i'm not too worried about the whole thing