Case fans are mainly there to be the Case Temp down. It does effect the CPU temp to some degree, but not by much.
If you want to keep your CPU temp down, then get yourself a good heat sink.
I am currently kicking a Thermaltake Volcano 11+ with an adjustable fan speed controller.
How ever, there is nothing stopping you from slapping on a 120 mm fan on the side of your case (some thing my sister's BF did on his computer)
When it comes down to case fan, I tend to put some sort of filter on all the inward blowing fans to keep the dust out.
If you are trying to work out what you can use for a filter, then I can suggest that you can use some of the phome padding that comes with your motherboard box. It will reduce the dust, but it will let the air do through. Although it not that nice to look at if you have clear see through panel. (But then again I have a bit of bias against those sort of mods.)