ATI Radeon Vs NVidia Geforce

[Nerve]

I know that in all emulators what really counts is the clock/architecture of the CPU you're using in order to get better speeds, but since both the Wii and the Gamecube use ATI graphic chipsets, wouldn't there be a positive difference if we use a Radeon? Also, will it make a difference if my graphics card has 1GB of video ram (dedicated, of course) instead of 512MB? Thanks in advance.