You will need to remove all the panels you can from the actual chassis. You will want to tape off the parts that slide into place or have fitting parts - paint will make it thicker, and the parts won't fit back together. Also - be careful around the ventilation holes - paint will clog them up easily....
You will want to scuff up the paint on the existing case with sandpaper to make sure the new paint adheres properly.
For the best results - I would tape off an area to paint that doesn't have ventilation exposed or has parts that would connect to another part....kind of like accent striping.