if you upgrade the crossbow, when you shoot it, the zombies get attracted to the flashing light.
if you upgrade (i think) the spas, it freezes any zombie in its range.
the most useless upgrade i have ever done is the ray gun, = same damage (maybe a bit more)
Thunder Gun which turns into a Zues Cannon. My favorite gun to upgrade in my opinion is the Aug. You get the same amount of ammo per clip but it gives you a shotgun attachment that works really well for the nova six zombies. Plus the sight makes it easy to hit zombies in the head.
