Let's say I have a graphics card with 256 megs of memory and my system has 4 gigs of RAM. What happens if I try to fill up some VBOs with a total of 1 gig worth of data? Can I expect that my extra system RAM will also be used to store these VBOs or is it illegal to allocate more memory in VBOs than my card has?

I'm wondering if I should load all frames of animation for a particular model into a VBO or if I should keep the data in main memory and then use glBufferSubData everytime the frame of animation changes and store a single frame in the VBO.