As others have already posted, the Saffire 6 works just fine with Sonar X1 and Windows 7 64bit.

The Saffire 6 USB should work just fine with USB 3.0 provided the latest version of the drivers have been installed.  If you do decide to get a Saffire 6, make sure you install the latest driver before connecting the Saffire 6 to your computer.  
If you would like us to review your computer's spec. or have any specific questions regarding your system with a Saffire 6, submit a case through this link:
 http://www.focusrite.com/answerbase/en/ 

I picked up a Saffire 6 USB for an interface for my laptop. Needless to say, I had to try it on my regular studio desktop. I was not surprised to find that the unit performed wonderfully! Great detail, very warm to the ear. And the included software was a great plus as well. The only qualm I have is this unit is USB 1.1! I heard that the newer drivers allow it to use USB 2.0 ports but I am not convinced that the latency would be a little faster if that were so. The latency had to be set a little higher than what I achieve with my MOTU 24io. But, I still say bang for the buck-wise, it is worth the investment!

Just another +1 for Saffire 6, I've used it for close to a year, and although work on smaller projects, never had one problem, and running same specs. as you. I love it!

Sonar artist 64bit  hangs with focusrite saffire 6 usb. any suggestion?
Thanks

try reinstalling the driver. I used a Saffire 6 for a year or so back in early 2011. It was solid as a rock.

zplicit
Sonar artist 64bit  hangs with focusrite saffire 6 usb. any suggestion?
Thanks

If you're running Win 7 or 8, I suggest doing all of your installs with "Run as Administrator".  Also, try a different USB port (Make sure it's USB2, and not USB3).