Have you checked out this list by Bulbapedia?
http://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_effort_value_yield
It's a really nice, comprehensive overview of which pokemon to faint to get what kind of EVs back for your pokemon.
(Sorry, you'll have to copy the link as I'm having trouble getting the URL to work with the é.)
I'm not a battler to ask for advice, but I simply pick two EVs I might want to train - as in SPATK and SPD in a Gengar, so I focus on those to the maximum and put the leftover bits on another stat, maybe HP or SPDEF. Maximum EV for one stat is 252 - 4 EVs are needed to gain a stat point, and 510 is the maximum in total. Also, you've probably seen this if you started reading up on EVs:
http://bulbapedia.bulbagarden.net/wiki/Effort_values
Hope this helps a little.
I wrote this before you edited your post, so yes, it is basically a stat boost for those stats most important to your pokemon.