What is the most important thing to look at when getting a video card? Is it Core Clock, Shader Clock, Memory Clock, Engine Clock?
Meh, don't look at those things individually. Even if one card has a higher core/shader clock, it's possible that it's slower than a card with lower clocks but a higher number of shader units/TMU/ROP. Another thing, not all games are made the same way. Some games might favor faster shaders while others faster texture mapping. You really need to check reviews to see how various cards perform on a certain game.
lets say if the vc is mainly for sc2 what specs would you look for? or the number and letter abbreviaions on the cards, do the signify anything specific?