Hot air rises, so it really doesn't matter with a single fan. Blowing up will more or less cause the air to rise in the center of the room, and go down along the walls. Blowing down will do somewhat the opposite. Depending on whether or not you have A/C and if the supply vents are floor or ceiling mounted. Just the fact that air is moving will help. If you are concerned about personal cooling, an oscillating fan is my recommendation.
Experiment I found that having my fan blow air up makes the room more comfortable personally. But during the summer that doesn't do enough and I need to put in a window fan blowing air outside to get rid of the hot air inside. So experiment.
Ohh and one last thing computers don't help much in keeping the room cool. I know my computer keeps my room at least 10 degrees hotter than all the other rooms when its running and I'm gaming. Helps during the winter though