Only Amiga models with the AGA chipset, or at least ECS can create other modes than the built-in. I.e. A1200, A4000, A3000, A600. But not A500(+) and A2000.
However, according to your first post, you already have a FlickerFixer. So NTSC Hires Interlaced 640x400 is the mode to choose. All other modes can be displayed without a FlickerFixer anyway. Choosing those already-flicker-fixed modes can damage the FlickerFixer. You won't get higher resolutions than that.
Bye,
Thomas
