It would make a diff if your expansion cards get hot. If you have a graphics card with a passive heatsink it would help. Otherwise i guess it would help airflow a bit but if your case temps are fine you'll just be adding extra noise
It does, but they clog too easily and are difficult to brush clean. If you get a dust protected case (expensive!) you can use a fan like that to great effect. They do however add to the system noise...
Rather get yourself a Vantec Spectrum Card. They have two fans on that blow directly onto whatever card is above it. They are very very efficient too...