Few things i thought i'd go over
1. You SHOULD sand the X part of the case. Since it has a texture to it, without sanding it doesn't look as good. And the only other thing you can really do is spray the X with four or so coats of Filler Primer first.
2. Clear coating the case is more to protect it then anything. Even though done right, it add's a lot of depth.http://case-mods.linear1.org/case-mod-101-...er-case-part-1/http://www.mnpctech.com/case-mod-paint-com...mod-how-to.html
3. You don't NEED plastic paint. only thing you might want to do is put a coat of plastic primer down, before your filler primer, or sandable primer. just don't see how much you can flex the panels on the xbox and you'll be fine!
4. Take your time! Not like you'll be doing this every day! Don't try and take the easy way out. You want a show piece that makes your friends hate you, and other modders pick your brain about how you did it.