Extra memory won't help graphics performance--there is no way to get Aero to run under virtualization. The reason for this has to do with how graphics are implemented in a virtualized machine--Windows running under virtualization is NEVER allowed to access the physical graphics card directly. Instead, Windows talks to a "virtual" graphics adapter, which is actually a software layer that takes the graphics commands from Windows and translates them to something that can be integrated by the host OS (in this case OS X) into the commands that it is passing to the physical graphics card. All of this adds a tremendous amount of processing overhead, which ensures that graphics-intensive operations like games or Vista Aero won't run too well (or at all).
VMware covers this in one of their FAQs. There, they point out that the only conceivable solution to this problem would be for graphics cards to be expressly designed to deal with two (or more) different OS's at the same time, which would allow the guest OS to directly access the hardware. No such card exists, at least commercially, today.
