I have no idea about design specs, etc.
I can only tell you my personal experience running Vista on a Sony SZ460 with GMA950 graphics, and it always ran smoothly and just fine even with many windows open and using window previews in the taskbar, Flip3D, etc. and even having video playing.
Either way, GMA950 graphics shares system memory (usually it would take about 200 megs of my 2GB of memory when running with the GMA950 graphics), so I don't know how big a deal the issue is with storing stuff in video memory anyway.
I am new to Macs and this forum, so can you tell me how I can upload some screenshots to this thread? If so I will show you what the performance results are for this. It is quite easily seen in the performance score for the gma950 which I have here and can show you.
BTW.. Your right it does share system memory which is many times slower than true vram and also takes that ram from the OS which has an impact on the interface as well. But thats another story. It is no comparison to true video memory. The GPU part of the GMA950 is yet another area of weakness and it is about at the performance level of an old 1st gen Geforce circa 1999, but has no T&L capability so it is actually weaker than that.