you've got this great mountain landscape photo, and it's all tweaked the way you want it and you're going to move it into Painter to auto-paint, why, because I don't currently know anything else.

what settings do I use on this photo in Photoshop to work best in PainterX

Before moving to Painter for Autopainting, in Photoshop I would apply:
* Image > Adjustments > Hue/Saturation - and increase the Saturation
* Image > Adjustments > Levels - to increase the contrast

thanks Danny,
should I drop the size down to around 75 ppi or leave it at 300ppi?

It depends... if for monitor display only, 72-96 ppi is fine. If you intend to print, 200-240 ppi usually works for "arty stuff" unlike photos which usually call for ~240-300 ppi.