The current version of the X1 Professional Client is v6.7.3 (build 4054dh), released May 13, 2011.
You can always check online to find the latest version of X1 by visiting any of these pages:
Release notes page: http://www.x1.com/support/release_notes.html
Frequently Asked Questions (FAQ): http://www.x1.com/support/faq.html#1
Current customers can log into your support portal account: http://www.x1.com/support/portal/
To check your X1 version, simply open the X1 Professional Client and select Help > About X1.