They are the same importance, as far as Aero is concerned. You could buy a $500 video card, and you still couldn't run Aero.
You could also upgrade to 12GB RAM, and you still couldn't run it.
You could upgrade to 12GB RAM and a $500 video card, and you still probably couldn't run Aero, because your CPU is not powerful enough.
See what we mean? Your machine simply lacks the power. You are best off buying a new machine, or just using your current one without Aero.
EDIT: I just paid $50 for 2GB RAM.