no, it still 512mb. GDDR5 just means it high speed RAM.
it depends what resolution you are at, but the ammount of RAM is not that important in deciding the performance of a game. the onloy real impact is when the game needs more than the available ammount of RAM, so performance degrades drastically.
but performance won;t be that great with that card, because GTA4 is a horribly coded port. at a low resolution, you will be able to play fine, but anything too high will require substantially lowered settings.