I know there has been some debate on this topic and I happened to come across a nice little program that displays and records how much Vram you are using. Should be a decent tool to decide how much Vram you really need for your games.
PS This program only works on XP. It would give a error in Vista after a few seconds. Might work in 32 bit Vista I don't know I ran XP SP2 compatibility mode and it wouldn't even launch.