Heh, finally someone made a texture, which would make a girl look like a girl, not like a ghoul
I can't really see much from the picture with mutie, but from that distance and this angle, it looks quite similar to the original. I personally trust you very much with all the things related to the characters modelling, coz you had shown your skills with your previous models.
And yes, most probably, you can use 512x512 at this stage of development. Texture size could be reduced at other stages of development.
P.S. Don't worry about exporting to .x format. Just export to any other file format, when you're done (though, try to avoid specific file formats, like
.blender or
.wings)