Dual-booting is when you have two operating systems installed on your computer at the same time. This lets you have access to multiple operating systems with full hardware access for each. You will only be able to boot into one at a time though, so it's not perfect.
Make sure that you have 3D acceleration enabled in the video settings tab:
Also, go ahead and crank up the video RAM that it's giving it.
Looking at VirtualBox's descriptions of what these options do, it looks like they might actually pass directly to the physical card. Hmm, I'm gonna have to play around with that later...