Cant adjust VRAM on Parallels

Hi. I recently purchased GTA IV off of steam. I downloaded the game. When I went to play it I received a message saying that there is not enough video memory and to restart the computer. After looking online for hours I figured out what I needed to do in order to fix the problem. When I went to adjust the VRAM I noticed that I can only go up to 256mb and not the full 512mb or less. In order for me to run the game on full settings I need more than 400mb of VRAM. How can I adjust the VRAM so I can get the application to work?
If I need to reinstall Windows 7, is there a certain way I need to set it up to share the VRAM or select how much VRAM I want to give to each when parallels is running?

Thank you for your help

 

The maximum VRAM that you can use is 256MB. I'm not sure why Parallels won't allow anymore considering 512MB is what low end video cards come with.

 

If I get a new Video card will that fix it?

 

If I get a new video card will that fix it? Also Is that even with parallels 6? I have parallels 5 now.

 

Parallel 6 supported 256 MB max also.