Heh. I'm continuously amazed at the disinformation wrt dust accumulation. Without intake filters, positive vs negative case pressure is immaterial. It becomes a function of environmental cleanliness and total airflow- the dirtier the environment and the more the total airflow, the more dust and lint that will be deposited in the case... the computer acting like a low efficiency air purifier... most of the dust that comes in goes right on through, anyway.
And, yeh, lots of people will tell you that 500W isn't enough- that's because they're accustomed to cheap over-rated psu's whose continuous output is more like 300W, even if they're rated at 500W. The reason manufacturers can get away with it is because you really don't need 500W in the first place.
The one disadvantage to the slk3000 is the small intake area on the front bezel, which will lead to some recirculation if a powerful front fan is used. The way to avoid that is to not use a front fan, or if you really need one, to block off the passages that allow for case air to get back out between the case and plastic bezel... the fact that it has large side openings for airintakes more than makes up for any lack of front vents, anyway...
You'll need to plan on replacing the chipset fan with a zalman unit, and the gpu fan, as well, anyway, if you actually want quiet, along with a suitably low noise hsf on the cpu. It's nearly impossible to kill the high frequency noise from whiney little fans, no matter what case they're in- much better to reduce noise output up front, and then select a case that breathes well so that wimpy low noise case fans can be effective...