If I can't afford another lens for my camera, should I invest in an extension tube set?

I recently purchased a canon digital rebel xt, and I current own a sigma 70-300 and sigma 28-80 lens. I would like to be able to zoom in closer to objects, and I read in a photography book that extension tubes can many times help your focal length lens focus closer to objects. I cannot afford $350 for a new lens, and I would really just like to improve the power of the 70-300 lens. Would an extension tube be the route to go? And would it work with the sigma lens?