The pixel clock is number of pixels (horizontal resolution + horizontal retrace) * number of lines (displayed lines + vertical retrace) * refresh rate. For example, NTSC low res is: (320+135)*(200+62)*60=7.16MHz
The Amiga has three possibilities low res, high res, super-high res (28.6MHz NTSC, 28.3MHz PAL)
720x480x60hz or 720x512x50hz plus a bit of overscan is about as good as it gets without interlacing. As previously mentioned, you can get 800x600x70hz interlaced on a 24KHz monitor and the flicker probably isn't too bad (at least compared to a 15KHz monitor).
Since AGA can do 8 bitplanes in super-high res (with 64-bit aligned fetches) it can do it in any possible resolution, but CPU-chipmem bandwidth gets progressively slower going beyond 4 bitplanes in super-high res
