In theory, the GMA950 can access 224MB VRAM max. It is supposed to dynamically allocate what it needs when it needs it but who know what limitations Apple may have put on them. I experimented with an overclocking utility for this chip on my MacBook. It had to be done in Windows and made no permanent difference when I rebooted to OS X.
I do recall reading the clock speed of this chip is higher in a Mac Mini than it is in the MacBooks....

http://www.intel.com/products/chipsets/gma950/