What video card should I buy for PA?

Check if your motherboard have latest available BIOS, if not try to flash latest version.

How smoothy it is? I have one 7970 and it only runs around 40 fps on uber setting at the game start and drop to 10 fps when many units are on the screen. And GPU load is only less than 70%, I posted on steam about the performance issue and they told me that it's the steam community overlay and the UI system in the game cause the bottleneck.

PA runs on any hardware.
upload your dxdiag, get the latest beta drivers and make sure .net dependencies are installed (though windows should launch the installer automatically upon launching pa)