Well first things first you might need to get some sandpaper and sand down that case depending on the finish already on it.
If you have a powder coat texture finish you'll want to remove it, the new paint will stick better to it this way, and remember to use different grades of sand paper so that its not too scratchy once finished. Once you've done that clean the case thoroughly and remove all dust and apply some paint thinner.
Make sure you cover buttons or anything already on there you don't want to paint over, I learnt from that mistake when I thought just spraypainting over the front would be fine, turned out power button got stick, couldn't turn the **** thing on.
Now I'd suggest you go with a black or a grey to match the front of your case. The simplest thing to add to your case and what I started off with when I first started moding was flames.
Get some masking take and some very thing line tape and just make a general shape of a few flames along the case and take different shades of orange and red and spray away. Start with the bottom of the flame first.
If you're worring about messing it up test it on a piece of wood or something first, but it's honestly not that hard to do its just lots of people are worried about messing up their case and being left with a terrible paint job.
Side windows are common but still look good, you've got the choice of showing off whats in side are lighting the insides up with some cathode lights which will pretty much blind anyone who tries to look inside the case as well as provide you with bedtime lighting.
But take it from me blue LED fans and lights inside can get annoying.
You try sleeping with 6 blue LED fans and 2 cathode lights lighting up a room, provides more light than my cieling light and lamp put together.
It's hard to really give ideas of how to mod and explain it thoroughly but I hope I've helped you get started, just use your imagination and if you're afraid of trying something straight away test it out before you do it on the real thing.