Hmm, I thought this said Suck or Swallow then...
As for the actual topic... hmm... they can do either, it depends on which way the blades are pointing.
My aging Athlon XP 2000+ has the fan on the HS and I have one in the side window, the fan in the PSU and another in the back wall. Reasonably quiet but the speed control is turned right down- just as well as it's right by the bed - and left on! Have a 64-bit jobbo on the way, ordered the parts earlier
I would imagine that cooling would be enough. I don't want to buy more anyway seeing as they look good, nice display of blue flashing lights which I can't turn off...
[My typing's gone silly, so sorry if there are any typos I've missed. It's dark]