Well, just wanted to post my results, the RivaTuner softmod successfully worked on my MSI 4870, not only was i able to raise the voltage but it also resulted in higher clocks. I was previously stuck at 815/945 @ 1,263v but a voltage increase to 1,2875v allowed to up the clocks to 820/945 for now. May be i'll be able to push it higher at that voltage, who knows? It was a stable overclock too, it passed a 2hr. ATI Tool artifacts scan.

However, this obviously resulted in higher load temps while stress testing (core jumped to 52-54C, MEMIO to 62-64C, VRM to 62-64C) so i'm not sure how far i can push it with the stock cooler. So I have Thermalright T-Rad and copper ramsinks standing by and i'll be using after the stock cooler is pushed to the limit.

And finally, a question, anyone here knows what the highest safest GPU vcore should be for 4870 cards?