This will depend mostly on the power of your MacBook. Most MacBooks with integrated graphics won't perform this very well, if at all. What sort of MacBook are you using, and what USB graphics cards do you have in mind?
I've found the latency isn't too bad over native FireWire, but using the capture card might add more. You could always go the old fashioned route using a VCR as an input switcher, or a simple RCA switch between a DVD playing a black screen. Your switch becomes the on-off switch for the video.