sheet DPS = the DPS shown on your character sheet in game or profile
EDPS = your actually damage output usually calculated by noting a monster's total HP and dividing by the amount of time it took you to kill said monster. If all you are doing is spamming magic missile (not that anyone would do this) your EDPS is very easy to calculate. It would just be your sheet DPS times 1.25 because MM hits for 125% of your weapon damage and it has no resource cost. But for a build like SNS, it is much more complicated and must be done by timing a kill of, say, Ghom.
EHP = your effective hit points = HP/(1 - mit)
mit = mitigation your total damage reduction = 1 - (1 - ArmorDamageReduction)(1 - ResistDamageReduction)(1 - OtherDamageReduction)
Example: if you have 65% DR from armor and 70% from resists and 7% vs elites your total mit vs elites would be 1 - (1 - .65)(1 - .70)(1 - .07) = .90235 which would usually be described as 90.235% mit.
I have a storm crow with LOH and CC, but no all resists, should i use that instead of mempo?
SC only has 1 random mod so CC is the best thing you can get on it. The same goes for mempo. The difference is mempo has great EHP while SC has 2 utility stats: LoH and APoC.
If you are going to use storm crow, I would advise that you pike up a little more vit from your force. Go for split int/vit or a cheaper option is straight vit for the 1 random roll that chants force has. For split int/vit, just search for a force with average damage (most important stat and it allows you to sort the results by damage) 201 int, and any amount of vit. That will give out only split int/vit forces. If you want straight vit, just search average damage and vit (put in a min of at least 160).