I also own a 6950 (MSI). My personal methodology is that both the voltage regulators (VRM) and core need to stay at 85 celsius or below. I use GPU-Z to monitor them - however do note that using GPU-Z (maybe other temperature monitors also) causes a small hashrate drop and thus temperature drop, so my policy when I was balancing the temperatures on my mining setups was to keep everything below 75c on GPU-Z while it's been open for a few minutes. Once GPU-Z closes the temperatures seem to go up a bit but 10 celsius seems to be enough of a margin.
You could also try -w 256 but if I recall correctly it doesn't make much difference on mine. I personally don't use GUIMiner anymore as it doesn't seem to get as much development these days. I recommend you checkout ckolivas' CGMiner or DiabloD3's DiabloMiner, which get regular, sometimes daily updates.
I find MSI Afterburner to be the easiest tool for modifying core clocks, fan speed, voltages, and memory clocks on ATI/AMD video cards. CGMiner also supports some of the same features but I don't know how to use them as Afterburner does everything I need.
Hope that helps mate.