I have a Zotac GT 430 2GB DDR3 with a measly 96 cuda cores that I can add to my third PCI-e slot, will that work?

Yes.  The display adapter doesn't need any cores! 

Your 580 is a 1.5 GB of ram which is a very small amount, I gather 3 & 4 is more appropriate but does that extra 1Gig between the 580 3G & the 680 4G makes such a big difference?

I would have thought it would be very rare to use up 3Gig, so 4Gig seems like a marginal difference.  I have 1Gig, and that's fine for the renders I do, but I have troubles loading some of the scene people send me into that space.  You probably can't buy 1Gig cards anymore!

Creator of PoserPhysics
Creator of OctaneRender for Poser
Blog
Facebook