when I painted mine, I used duplicolor paint, it's automotive paint and it worked great.
wetsand your case with 1000 grit sandpaper first to get it nice and flat. make sure you sand lightly so as not to go through the existing paint to the metal.
prime it then wetsand again. prime again.
apply your colors, paint all your parts at the same time in a room with little dust and humidity. If you are doing it in a garage make sure you sweep the floor, and try wetting down the floor to keep any remaining dust down. don't want anything getting in that nice paint.
after the color is applied and has dried for at least 15 mins...apply clearcoat to all the parts...as many coats as you like. trust me, it will make things look a lot better, and if you want to make it look even better when it's all done, get some light rubbing compound and rub all the parts out. it will make them shine like crazy.
make sure that when you paint...you take EVERYTHING off or out of the case that isn't going to be painted, motherboard, psu, all hd's...everything! you don't want to get paint on that stuff. make sure you tape everything up good so that no overspray gets where there isn't supposed to be paint...makes for a messy job.
make sure it's at least 70 degrees F outside with low humidity, helps the paint dry better. and make sure that all the parts are completely dry before you reassemble them, I'd let them dry for at least a few days before I'd reassemble them.
if you use a dremel to cut a window in the side, or to cut anything else like a hole in the top for a fan, make sure you practice a little before you do it, when I did it, I had some metal that was about as thick as my computer case and I could practice before I did it for real on my computer case, you don't want to screw something like that up because once you cut it, it's for good (unless you have a welder or something...lol)
if you want to do graphics of some kind, you can buy vinyl overlays, or you can just cut some out yourself from vinyl sheets.
here are some pics of my mod job.
some of the pics are crappy...sorry for that....crappy camera.