I would suggest you start farming materials now for +30 spell power, but maybe save them for a weapon that you'll use longer. The Soul Harvester isn't likely to be your last non-epic weapon, so unless you really like the style (and it is very stylin'!), I'd consider something else for an expensive 'chant like that.
Of course, if you're feeling rich, you could enchant this weapon and your next. Either way, I'd stay away from +int enchants on Warlock gear. Spell power is far more effective for warlocks than any other class, because it applies to each of the several DoTs that you'll have ticking simultaneously. Because you're typically stacking three or more dots on a mob, that +spell damage is working overtime for you.
Because stamina is so important for Warlocks, the
Whiteout Staff from AV is a really good choice, with nice high stamina and decent +dmg (to all schools). It takes some work to get the AV faction required to purchase it, but it's worth it.
If you're looking for something a bit more balanced, the
Rod of the Ogre Magi has better damage and 1% spell crit, although you give up some stamina. It's easier to get than the AV staff, since it's a reward from DM tribute runs.
And if you're the sort that prefers damage over everything else, go grind Abyssal Templars in Silithus until you get an
Amethyst War Staff to drop. With a spell power enchant, it's a tasty +64 to all schools of magic. :D
The Amethyst War Staff with spell power is the top-end butt-kicking option, IMO. With a spell power enchant, it will contribute at least 180 more damage per fight, just in damage from CoA, Corruption, and Immolate. That's far better than 330 additional mana from the +22 int enchant.
~Kv