I would make the upgraded one cost 3
as well, and make it do +2/-2 instead of +2/-1.
Since this is a card that will be used with high hp creeps anyway (basilisk blood, stone form gargoyle, flying titan, you name it), this will still be powerful. It's actually like acceleration, but FASTER! =D
Also, I'm not a great fan of the upgraded name (Overboost). I guess I'd like Overdrive better.
I do very much like the name Acceleration, since it matches very nicely with gravity and momentum in providing that real physics feel to the element (and to the science feel of the game in general).