I measured mine with a temp probe and it quickly went up to around 50C, now i have added this 120mm fan, running at a very low rpm (almost inaudible). It is suspended by elastic and blows air over the NB and my passive graphics card heatsink. The NB and graphics card now run much cooler.
So it should be fine, but a small amount of airflow is a good idea.
It looks like you have the same if not similar setup to me regarding heat sink and video card. Is yours the Gigabyte card 6600 GT (which already features a Zalman heat sync?).
I will see how it goes. I have a 120mm fan on the front of my case, which blows air pretty much over the northbridge, so hopefully that will do a job on cooling it enough. If its not getting cool I may look at moving the fan to a more prominent position for the Northbridge fan.