Fan at the bottom, fan at the top? That's how my cab is and runs cool.
+1 on this and working with the tendency of hot air to rise.
Fan on the bottom blowing cool air in.
Fan on the top blowing hot air out.
Make sure that there is enough vertical separation between the fans so that the hot air exiting from the top isn't pulled in by the lower fan.
Scott
EDIT: To reduce the amount of dust drawn in, use a dust filter like Kevin mentioned and place the lower fan ~6-8 inches above floor level.