When I switched from an old X700 to an HD5450 I noticed a definite improvement on the colors and, unless my poor eyes deceived me, the sharpness of the image.
I believe that newer cards have better DACs on board that will give you a better image on your monitor.
I don't see any difference myself, comparing X1050 and HD6450 here. Maybe I should pull the oscilloscope out, but many things to do.
Most video card since about 2006 have used the same 400Mhz DAC (RAMDAC), with a few exceptions. There is no incentive to update RAMDAC in modern video cards as it is not needed for modern display types. I'm pretty sure the X700 cards used 400Mhz, though they might come a couple of years earlier.
So I think it doesn't really matter.