The 50 MHz wont do much by itself unless you're using very high resolutions where the RAMDAC becomes the limiting factor, which wont happen until you hit veru high resolutions and refresh rates.
However, one of the RAMDAC's may be of better quality than the other, which will make a difference.
Faster doesnt mean better quality however, a G200 for instance has "only" a 200 MHz RAMDAC, but its of very good quality and provides very good 2D quality, assuming you have a good monitor.