Adding a small sub to augment a decent car stereo?

I'm looking to spend $100-200 adding a 7-10" sub to complement the JBL system in my '05 Tundra. Everyone says you can get better results picking out a sub and an amp separately, but how much improvement is realistic to expect?

I do not want a system that's so loud the car two lanes over is vibrating, I just want to add a final piece to augment the stock system. I have plenty of room now, but I'm thinking about downsizing in the next year or so, but how big can the box for a 10" really be? 

I don't know anything about this sub-field of audio and it'll take some work to begin even basic research, so if I'm looking at only a modest improvement it probably wouldn't be worth it. Whatever way I go, I want to be done with it by the end of next month. 

So, just plop in an Infinity Basslink or a Pioneer KSC-SW11 and hope it's an antidote to upgraditis, or take the time to do it "right?"