I disagree. I dont' think any 512MB card can max out GTA4 at any normal resolutions. (800x600 maybe...) The view distance will kill the frame rates.
How much will you be getting? Try it and tell us.
Edit: When it came out Hardocp did some testing of the game and a couple of different 5870s including a 512MB version of that card. Its a game that likes GPU memory.
View distance in game has more impact on the CPU than the video card. The greater view distance the more objects the CPU has to render. Games like GTA IV with many objects are often more dependent on the CPU because all of those buildings that needs to be rendered in the background.
That being said I can't say if that video card is good enough for high settings in GTA IV