Wobbly headset
 

I switched out my stem on my SST AL and the headset is wobbly for some reason. Is the stem suppose to be higher than the steering tube or is it the other way around?

I accidentally overtightened the stem cap and the cap broke. The screw isn't stripped. Will a new cap fix the problem?

Yes, you need a new top cap. The screw into the top cap is what sets the preload for the headset. Why were you tightening it so much in the first place? Don't ride your bike until you fix this.

http://www.sheldonbrown.com/headsets.html#threadless

After you get a new cap, tighten then check the bottom of the headtube. I had a problem with everything settling into place evenly. I usually butt the front tire against a wall and grab toptube to shift front and back. Once bottom of the headset is aligned tighten everything properly. And make sure stem is loosened before doing this. Good Luck

Your new stem's clamping area is probably a few mm shorter than the previous one. Get the appropriate headset spacer(s)

And you probably need a new stem cap.

Both of which are cheap and can be found at any local shop.