I would not wanna use the stock HD4850 cooler with 1.26v. You are putting about 35watt extra trough that heatsink. The HD4870 are doing 1.28v under load. And that massive heatsink is getting very very hot.

So i suggest better cooling for 1.26v.

I have a VF1000 on it now. Still hot 62c load with 1.23v 775-1125. I'm gonna do the memory mod this week.

The HD4850 on 1.25v is consuming 5 watt more than the HD4870 on 1.28-1.29. So that GDDR 5 uses 10-20 watt less than GDDR 3.

I'm not gonna clock the core much higher because at high resolutions it will get memory bottlenecked anyway Maybe not in 3DMark but in games the faster GDDR5 is showing boosts up to 25% with HD4850 and HD4870 core both clocked at 750 MHz.

BTW power draw from 1.18v to 1.23 increased with 25 watt. Compared to stock 1.14 you have a increase of 35-40 watt. 1.25 is on par with the HD4870 stock.