Nvidea GeForce 8600M GT card and memory leak

Nvidia does not release official mac drivers, Apple does. You get all your driver updates through Apple Software Updater. You might try doing some graphics card emulation using OpenGL Profiler and chose one of the cards in that list. I honestly have no idea if that will fix your problem or make it worse, but you can give it a shot.

To use OpenGL Profiler you will need Developer Tools installed and it is in Developer>Applications>Graphics Tools

The first time you start it up you will have to do a small configuration thing, it is not hard to do. Then you will see a window that in the upper left has the option to launch or attach to an application. Choose launch, and add the unity application in there, and then expand the launch settings part of the window by click on the triangle. You will see a drop-down that reads "Native Graphics Driver" Change that to the most advanced card on the list, you may have to look them up to find out. then launch Unity with it and see how it performs.

I looked at the link you provided and that is not for your graphics card, and it would be bad to install that onto your computer.

That would mean that you don't have it installed. To install it you can download the latest Xcode installer from apple's site or do it off your OS X install disks, then update it.

Yes they do make mac drivers but they do not put out the official releases for them. Apple controls that aspect of the updates. All drivers for the hardware in Macs are managed by Apple, all updates OFFICIALLY go through them.