The RAMDAC limits the maximum output quality nothing else.
(e.g. for 1920x1080 @ 60 Hz ~ 135 MHz pixel clock you'd need at least 270 MHz RAMDAC for a somewhat decent display)
A bit more exact: The RAM part of the RAMDAC limits the maximum pixel clock in palettized modes - this usually isn't a problem -, but for hicolor or truecolor modes it's just the DAC part that matters.
