General rule is front/low fans pull air in, while back/high fans suck air out. Side fans can be found doing either. This is because warm air rises, so you want the fans higher up blowing out. A blow hole fan at the top of your case should always be blowing out. I wouldn't worry to much about this unless your temps are high.
You need fans for both functions. The MUST be in balance - in fact, total air flow will always establish its own balance. You cannot exhaust air from the case without adding air to the case from outside - you can't manufacture air out of nothing! So what goes out WILL come in. Your task in setting it up is to help that process so that supply is nearly equal to exhaust. Then comes the great debate over whether the net balance should have slightly more intake (so the case interior is slightly pressurized and air leakage out of holes goes OUT of the box), or should intake be slightly less than exhaust so air leakage is INTO the box? I'll refrain from offering my opinion here, in the interests of NOT hijacking your thread into another debate.
Typically the air flow is bottom/front to top/rear. The fans at the bottom/front of a case are intake fans and the fans at the top/rear are exhaust fans. Power supply fans are always exhaust fans. A case fan on the side panel is usually, but not always, used as an intake fan. Depending on the fan's location it may provide additional cooling for either the video card or the cpu.
Scroll down to the bottom of this web page to see an animation of typical airflow in a modern case:
A bottom-mounted PSU is handy, particularly if your case has a grill which allows it to intake cool air. A dust filter is recommended however since obviously the floor or whatever your PC is on will be the dustiest part of your room.