The memory controller (obviously inside the core) will start to crap out when using too high core speed & voltage. At core 850MHz @ 1.350-1.375V (whatever I used) it becomes really fluky and my card (HD4850) will no longer pass FurMark stability test. 1.400V core doesn't help, neither does 1.450V, neither does 1.500V .

However the card is perfectly bench & game stable in other bench app's & games. Now the question is;

How smart is it to squeeze the core for the last 50MHz when the card & its performance is already held back by the "low" clocked GDDR3 mem?
These will not run safely beyond 1200-1250MHz.

How smart is it to create an even bigger memory bottleneck?

However, getting the core speed up to max on the HD4870 is a different story. The GDDR5 memory allows this card to scale properly with core speed up to at least 1000MHz (I'd think).

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I don't know, but I think I'll keep my HD4850 at ~800MHz core, 1150MHz mem (give or take 50MHz on both! ).