Check with Crutchfield (www.crutchfield.com) on replacement speaker sizing issues for your car. It's not only diameter that matters but depth as well.

The existing factory speakers are probably cheap paper cones. I think you'll find a dramatic improvement with almost speaker, but Infinity and Rockford-Fosgate are my favorites. Like Robert said, you might not get much improvement in bass with such a small speaker, but the highs and overall sound clarity will make it worth doing.

Pete