I haveto agree with the majority... by just adding a sub to your stock system.. you will drown out your highs... if you're going to upgrade.. I would start out with the HU... you will lose the stock HU usefulness but the sound difference will be nite and day....if not then start out by changing your interior speakers with maybe some alpine type Rs, infinity ref/kappa, rainbow dreams, etc...

Maybe you will be surprised by the midbass by further adding a 4 channel amp to the mix and if that bass doesnt satisfy you then you can add a small 10inch sub/amp to fill in the lower octaves.

Take it in steps... you will be able to save up money slowly and build up to a very nice system instead of cutting corners and only partially upgrading your system...