4670 Passive Cooler Overclocking

Hello,
What is the max safe voltage for a 4670 (Sapphire Ultimate HD4670 512MB DDR3) to overclock at? The "overdrive" tool included won't even hit 800 I think due to a very low stock voltage (unknown). I saw a review for the 4850 with 1.254v used but I don't know how safe that is or if it even applies to the 4670???

I couldn't get ati tool to work under vista 64 so I've installed an XP 32 disk I had floating around. I hope it fully supports 64 eventually....

Don't worry about heat issues, the CPU intake fan is right next to the huge passive heatsink sitting on a mugen 2 so temps under 100% load 3DMark 06' are 35C. The i7 920 only has a moderate 3.58 ghz overclock so increased intake air temperatures will be just fine, or worst case I'll ad another fan or 3.