SpellTips can display your actual spelldamage from items+talents in your tooltips.
http://www.wowinterface.com/download...Spelltips.html
edit: Sorry, I think I misunderstood your question
I don't think that there is a similar UNIT_ATTACK_POWER or UNIT_RANGED_ATTACK_POWER for spelldamage..
Look at the SpellTable.en.lua from Spelltips mod - it requires parsing through every talent/level and adding it to the original tooltip.