GTA IV for the PC is a fairly bad console port that is not optimized for the PC whatsoever.
The benchmark and auto-configure options are mostly useless. A quad core CPU has nothing to do with video memory. I think what you are referring to is the text on the bottom of the graphics options page that shows your GPU's VRAM and how much is being used by your current settings.
There are some things you can do to get rid of the vram restrictions all together, however this is probably not a great idea.