While you don't need a high-end gamer card to run SONAR X1's new graphics engine, a reasonably new series card with a good amount of RAM (especially at high resolutions) will provide smoother graphics.
Anything from the AMD 3000, 4000, 5000, or 6000 series should be fine as would anything from the nVidia 8000, 9000, 100, 200, 300, or 400 series.
What I'd look for is a card with 1gig of RAM at least, no fan, and multiple digital outputs (DVI, HDMI, or Display Port).
There's lots of fanboi-ism in the gaming world when it comes to video cards, but in the DAW world the brand of the GPU really doesn't matter. It's all about finding the right card with the right specs at the right price.
If you happen to have a preference of either brand, that's fine, but one really won't perform better than the other.
And X1 does not use CUDA or any part of the GPU for audio processing. If you want modified graphics cards that process plugins then get some UAD cards.

And then expect to deal with the downside of adding DSP cards to a native host, like latency.