After six pages of discussion over several months, I'm surprised to be the first one here to say:
DON'T DO THE PENCIL MEMORY VOLTAGE MOD!
However, the pencil mod for the GPU is great. It allowed me to get up to 750mhz reliably. My memory was even acceptable with stock volts allowing 1170mhz. But I wanted to give the memory mod a shot and see if I could push it a little further.
So I put a little pencil on the resistor, then used canned air to blow away the excess. Booted up the computer and immediately reached for the multimeter to check the voltage, when the computer shutoff. It didn't even finish posting.
I cut all power, and wiped down the modded resistor with some isopropyl alcohol, then dried it off with canned air. No boot. No power. DEAD.
The horrible part of this story is that this is the second 4850 I've killed with the memory voltage mod. I killed one last week by lazily shading the resistor while the computer was running (total bonehead move). Of course I know better than to do that, I just wasn't thinking. A quick trip to the local service center resulted in a card exchange. But now I've killed the exchanged card. Crap. I don't think the service center will be as nice the second time around since I was just there and they'll remember me. I could wait a few months, but I think I'm going to get a friend to go exchange the card instead. I hope that'll work.