It will not fix your issue I suspect but you can actually allocate more video memory by altering the memory allocated to 'Display VRAMSize' in the xml file that controls the VM. Not sure what a safe upper limit is but I have allocated 256megs quite successfully. Q111's suggestion would point to a solution though...
Originally Posted by matthewkirsch1
Edit: Mind you the manual suggests that an increase may very well be useful:
Video memory size
This sets the size of the memory provided by the virtual graphics card available to the guest, in MB.
As with the main memory, the specified amount will be allocated from the host's resident memory.
Based on the amount of video memory, higher resolutions and color depths may be available.