64MB is the default, regardless of the video card you use. 4MB video cards use a 64MB aperture. The aperture is "mapped memory" that acts like a buffer between the video card and the main system (CPU). It will take up address space in main memory, so you don't want it to be set too high. For performance reasons, you also don't want it set too low (32MB is low, 16MB is too low).
Search around on the net for aperture size in quotes. You'll get a ton of hits with explanations. ~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein