Nvidia cards need more ram for AA and higher res
ATI cards 48x0 up do it on the card it self so 512mb is norm big enough (thay fixed the AA on the 4000 cards at last)
most games will not use more then 500mb of texture ram (if it did it have to use system ram)
not tryed Quake 4 for some time does the Ultra setting on that game use more then 512mb texture set