reef, to be honest, I really don't know.
This issue has been debated a zillion times on many Forums across the world and as yet I haven't seen a conclusion that was proved positive.
I will say this though, most peeps agree it don't make any difference at all no matter what you set it to.
I was advised once, some time ago, to set it to half the video card RAM size, then again was advised to make it same size as Video RAM. Now I just set it to same size.
There are technical explanations as to what it actually does if you want to search for them, but I've found whatever you set it at, it makes no real world difference at all.