I did my first case paint job about a year ago. Here are some tips.
Patience Patience Patience while painting.
For a cool glossy look, use automotive sandpaper (wet/dry). Wet down the sandpaper before beginning. Start with a heavy grit (300 grit I'd say) and work your way up to about 700 grit i guess. Get it ALL smoothed down.
Paint it with a primer. Give it a nice, thin, even coat with the primer. Side to side motion at about 1 and a half to two feet away from the case.
Let the primer dry for the rest of the day.
On day 2, start painting with color. Again, paint side to side at a foot and 1/2 to 2 feet away in thin, even coats. Make sure you shake well often so that you dont get globs.
Should any globs or anomolies appear, use a very fine wet/dry sandpaper (wet it down) and lightly sand out the glob.
Put a coat on, let it dry for an hour, then add another coat. Keep repeating this process until satisfied. Even let it dry for another day and keep adding coats. Mine had around 7 coats.
Let it dry for another day.
Put a clearcoat on it! IMPORTANT IMPORTANT IMPORTANT! Just spray it on the same way you did with the paint. Let it dry, sand if needed, and you'll be done!
Here is an old Gateway 2000 case from 1996 that I painted last year:
This machine currently holds:
Intel Pentium 2.4C @ 3.2ghz
1g OCZ value ram
9800Pro (above XT)
Heh heh, and its for my grandparents! A dedicated folding machine for Uberclockers.com!