320MB VRAM

I have an 8800GTS with 320MB of ram and an E6420 overclocked to 3.4ghz. I know
3ds 2008 will take some real gpu power to do all those shaders in the viewport,
and in the trial I noticed some of my scenes go from 50fps in prespective to
10fps when I turn on best quality shaders and viewport shadows. Is this the gpu
core that is being maxed or the 320MB? 

The reason I ask is I have the ability to upgrade to an 8800GTX OR a q6600
processor. I know the 8800GTX has a slightly faster core but I don't want to
spend the money for the 15% core speed it offers over the gts 320mb. If on the
other hand I'm maxing out my 320MB, having 768 could perform substantially
better. 

My scene has vertex shaders, over a million polygons, and 4 lights. Is this too
much for smooth viewports on even an 8800gtx core or is it the 320mb I have now
that's giving me problems?

I know that gpu is for viewports and cpu is for rendering, and my priority is
viewports at the moment, but I don't want to upgrade for nothing so that is why
I want to know if an 8800GTS 320mb to 8800GTX 768mb is worth it. 

Also I play games on the side so I have a bias towards the graphics card over
the q6600, but I refuse to pay for the graphics card if it will not help in 3ds
and only games.