+40 SpellDmg will not affect crits, only SpellDmg. And while Int does affect SpellCrit, it's not a big enough difference in my opinion. (At level 70, it takes 80 Int for 1% SpellCrit)
As for the SpellDmg, it will indeed show as +40 on your character sheet. However, for all spells there is a formula. If you have +500 SpellDmg, it doesn't mean all your spells will do 500 more damage.
I suggest you check
this page to get an idea of the percentage of your SpellDmg each of your spells get. It's pretty accurate.
Just to sum it up, it depends on the casting time of your spell. Your spell must take 3.5 seconds to cast for it to benefit from 100%. Talents do not affect this so the talent which reduces Starfire cast time will have no impact and Starfire will still get 100%.
For damage over time, it depends on the time the DoT lasts. It must last 15 seconds to get 100% of your +SpellDmg.
In both cases, you can use a direct formula to find out the percentage for all spells. For example, Wrath:
100% - 3.5 seconds
XX% - 2.0 seconds
2 * 100 / 3.5 = ~57%