Here are factors to consider:
Overclockers are more concerned with airflow and evacuating the hot air from the case.
SilentPCers are more concerned with the noise the cooling system makes.
Case placement affects how much noise is too much noise. Is it on your desk next to your monitor or on the floor under your desk?
Case placement affects how much dust gets in the case and thus how often you need to clean it to keep all heatsinks and fans operating efficiently.
In a rig built for quietness, it is most likely the loudest item in the case is the GPU fan, followed by the CPU fan.
Going to a passive cooler for the GPU normally demands additional case fans to ensure optimal airflow over the heatsink vanes/fins. This is the trade-off for a passively-cooled GPU, and unless you use 120mm case fans with ducting, it is very likely your added cooling will be as noisy any decent aftermarket GPU cooler setup.
That said, many folks don't like the idea of having to modify the cooling system on a GPU. Some because they think it's silly to have to spend $500 for a top of the line videocard and then still have to spend more to mod it. Others because they don't want the risk of damaging a $500 piece of hardware. Either way, the ideal solution is a videocard sold stock with an aftermarket cooler on it, much like the Sapphire Ultimate X800XLs and the Gigabyte 7800GT, which both came with Zalman coolers on them stock.
There is also the Asus passively-cooled 7800GT, but there you run into the need to improve case airflow and possibly ramp up your CPU's fanspeed to ensure it cools properly.
Safe load temps for GPUs, imho, are anything up to around 60C-65C. Anything more and you're probably starting to shorten the lifespan of the card in a significant way and more likely to start experiencing artifacts during 3D gaming. When I see that the Asus 7800GT often loads up to 75-80C, I cross that card off my list. I can't accept spending $400 for a card that will require rearranging/adjusting the cooling and airflow setup in my case, will run so hot under load that the ambient case temp will likely rise substantially and thus increase the CPU's temp as well, performs only marginally better than my current GPU, and is a $100 premium over a regular 7800GT's price (which leaves it only $100 away from an X1900XT which performs infinitely better though apparently at the sacrifice of your aural sanity when it comes to the reference cooling fan design).
For those building a quiet PC, my advice (as I am currently taking myself) is to wait and see if any modified cooling solutions for the X1900XT come to market in the next 2-3 months. In the meantime just grab an X800XL or something equally as cheap to tide you over until then. Plus around May/June we should see the G80 series cards debut which will drive prices down on the 7800GTX and X1900 series (as well as everything below those GPU lines).
For those building an overclocked rig, go with an X1900XT since you can probably oc it a bit and it exhausts outside the case. Stick it under your desk and away from where you sit and crank up your headphones/speakers and enjoy smooth gaming at high resolutions. =)