Best Video card for Logic 512mb or 1 gig


I am a former pro tools on PC user who is switching to logic and mac for the first time. I know that on an audio PC it was always recomeneded to have a smaller video card since audio apps like logic and PT dont require a great video card. A Larger card (1 gig or above) actually hurts system performace of Daws on windows because they require more system memory leaving less for the audio app to use. So on windows xp if you had 4 gigs of ram and a 512mb video card , more of the 4 gigs would be avaiable for use by your Daw and VI's than if you had a 1 gig video card. I know with 64 bit this is less of an issue but i am still curious. So basiaclly on OSx if i have a 1 gig video card will that eat away more of my system memory than a 512mb video card. This is how it works on windows but im hoping that OSx handles it differently. I am looking at getting the new i7 imac which only has the option for a 1 gig video card. I wont be playing anygames or any video editing so i really dont need that much. Hoping that video card size does not effect avaiable ram on osX the way it does on XP.