Robin's build log may interest you:

http://www.mycncuk.com/forums/mill-b...ild-log-6.html

It's not an X3, but clearly the concept applies. Particularly interesting is how he pre-loaded the double ball-nuts to get (near enough) zero backlash. You could save money by doing the same with two standard nuts on the cheap ballscrews from China. It depends on what sort of accuracy you're after which depends on what you intend to make with it.