In two words, the big amount of guest VRAM is required to enable fast guest->host ogl data transfers over guest VRAM, to avoid extra memcopies, etc. This is especially critical for fast texture data transfers critical for Aero.
I have tested this some more and found that with 128MB of vRam as long as you do not change the screen size it works fairly well. But if you change the screen size using the (host+F) toggle or the min/max on the toolbar it refuses to draw the screen unless you save state and then start again. I set the vRam to 256 and it allows the screen resize (host+F) or even mouse drag to operate as it should.
System Ubuntu 10.10 x86_64 (no VT-x) 1024 base memory. win-7 pro x86_32, Beta-3 GAs, single monitor.