Yes, there is a point in going higher. It smooths the picture (namely, the edges) out. There's a reason there's both an internal resolution and a display resolution. An internal resolution of 1600x1200 with a display resolution of 800x600 effectively gives you 2x supersampling anti-aliasing. Unfortunately, it's easy to see the obvious in that you need a pretty large internal resolution just to get a meager 2xAA and at a small resolution, so "normal" (multisampling) anti-aliasing seems to have been introduced in the latest GSdx plug-ins, but you have to edit .ini files to edit the values.