That sort of memory is primarily (exclusively?) important for vids. The general tenet is to never play games on hardware you're making money from, but truth be told I find the only way I can sometimes get through a mental block is to blow away bad guys in 3D. So there you go, 64MB will make you more creative!
I run Vegas on a 16MB card and probably only use 1/4 of that memory. I'm running 1280x1024x24 bit. When i upgrade to a 19" monitor i'll be running at 1600x1200x24 bit and will be using about 1/3 of the 16MB. So you don't really need a lot of video RAM to run Vegas at all. Why do i have a 16MB card then? It's what was on the shelf at the computer dealer and it was only $60 as compared to $55 for the 8MB card, which was out of stock at the time.
But thanks, Doug, for a very insightful real-world answer :) you've got me chuckling for the rest of the morning!