Should both HD Audio & Video be disabled for SONAR Platinum to run smoothly?
 
Here's what I have:
Focusrite 6i6 2nd Generation
NVIDIA GeForce GT 635
 
I read somewhere that HD Video is okay.  HD Audio is not.  Is this true?
 

Hi. I have the internal sound card disabled. I have no use for it. Others have HD audio enabled to use with TV / Youtube.
All the best.

Drivers can potentially conflict, so unless they are truly used, disabling them can be a good choice. Most video data is punted to the GPU (if installed), so one will rarely see issues with video, but NVIDIA does have a lot of ancillary programs which have limited application with a DAW. An easier method of keeping this off the machine is to custom install NVIDIAs software, check the "perform a clean installation" and only select the "Display Drivers" and "PhysX" components. The other software in that package can cause conflicts depending on hardware/software installed.
 
You can also disable the HD audio drivers manually, so comes down to preference as well.

rgturner
I read somewhere that HD Video is okay.  HD Audio is not.  Is this true?

If anything, it's the other way around, although by and large it's not something you need to worry about.
 
High-resolution video is more compute-intensive, and you want your computer's resources to focus on audio. However, modern video adapters have their own processor, called the GPU, which handles the most demanding operations so your CPU doesn't have to. Still, it can be beneficial to turn off some video acceleration features in your DAW. For one thing, some audio plugins offload some of their processing to the GPU (e.g. Waves, FabFilter, Meldaproduction).
 
HD Audio just means studio-quality sound. It means the interface and driver can support 16- and 32-bit data, as well as multiple channels/data streams. If you're using the audio interface that came built-in to your computer's motherboard, HD Audio is both the default and the best choice.
 
If you're bypassing the integrated audio, though, and using a dedicated internal or external "prosumer" interface, then "HD" is the norm and usually just called "audio". Some DAW users will disable the internal audio interface because they don't need it (and doing so can avoid some uncommon problems). However, having it enabled is usually not a problem.
 
Bottom line: don't sweat it unless you're having serious issues with audio performance.

Thank you all! 

Mettelus,
 
Your suggestions did the trick!  Thanks!