1.55v is probably fine if you have decent cooling over the VR. I've ran 1.58v on my 4850 cooled by Accelero S1 without any problems for a few hours of benching

I'd like to see what the highest artifact free clocks you can hit on water are adpr. The 4850s seem to hit a ceiling at somewhere between 925-950MHz regardless of volts or cooling. Will run 3DMark fine, but fail ATITool artifact test and Furmark at those speeds. Download Furmark if you havn't already, its pretty heavy on the card, moreso than any other benchmark app I've tested http://www.ozone3d.net/benchmarks/fur/