First, happy early birthday!
I know that a good sound card will better the headphones if it is plugged into the back. I'm not sure about usb ones though, since those connect to the motherboard through the usb slots, as opposed to going directly through the card.
2. Would only work if they are the same model. Since it is an older one, I believe not.
3. I have two favorites right now. The classic Antec 902, and the Lancool PC-K62.
Both have leds. I prefer the lancool more because it adds an actual hard drive bay, has slightly better cable management, has a nicer looking window (opinion), and is totally tooless, using mostly metal designs instead of plastic, so it is actually a stable tooless case. They are both around 100 bucks.