I like to use Inkscape for vector artwork. It's great because it's free and it has a ton of features and capabilities. Krita is also a powerful program for illustration and image manipulation that I like to use (also free). It's vector manipulation is not as strong, but it is excellent for detailed raster images. It has a plethora of brushes, filters, and effects. As far as 3D renders, I like to use Blender (again, free). There's a steep learning curve on this one, so it will take some time, but at a $0 price tag, it's definitely worth it.
There's a host of programs that you can get for free to cover all your card designing needs. I use these three regularly in my workflow.