Like others said you should be fine with stock HS, however, you still need to make sure your case has good airflow, otherwise you will see high temps and possibly instability. If you are not going to overclock, then one fan in the front of the case blowing air in and another blowing air out will suffice. Note: You PSU fan does not count as an extra fan. You can monitor your GPU temps via the nvidia driver software. If you see your gpu is hot you could add side fanto blow on it if your case allows you. Or if your like me, use a dremel tool and make your own hole .
Just so you know, your GPU is naturally gonna run hotter than your CPU. And, in my experience, the cooling that comes as stock on GeForce cards are, shall we say, inadequate. You might want to think about upgrading that. As for the CPU, congratulations, the C2D needs no extra cooling if you're not gonna overclock.
Yarrrr! Make sure your case has good airflow and you're fine. (Air comes in the front and out the back - Side case fan for CPU and GPU are a plus) The 8800's hover at 60-70 C stock, at least the GTS series does under stress, but I haven't had any trouble keeping my CPU cool with a stock cooler while running this card. (CPU stays a cool 33-40 C under load with the GPU acting has a little space heater )
same here, my 8400 idles at about 57 with stock, and a side-case fan. And my CPU is currently 37, but that could be due to ambient. I haven't yet started my stress-testing, I'm putting it off till the cooler morning time. I'll let ya know.
EDIT: Silly me, forgot to enable SpeedStep and thermal controls. runs at 29 idle now. Still waiting till morning when ambient<25 right now its about 30 in this room. Stupid hot days...