Hit Command + N to create a new document. Enter 640 in the width box and 960 in the height box, then click on the Advanced button. Select RGB, Screen (72ppi), and make sure that the Align New Objects to Pixel Grid box is unchecked before you click OK. Now, turn on the Grid (View > Grid) and the Snap to Grid (View > Snap to Grid).
Next, you'll need a grid every 5px. Go to Edit > Preferences > Guides & Grid, enter 5 in the Gridline every box and 1 in the Subdivisions box. You can also open the Info panel (Window > Info) for a live preview with the size and position of your shapes. Do not forget to set the unit of measurement to pixels from Edit > Preferences > Unit > General. All these options will significantly increase your work speed.