Take the difference in price and spend it on RAM. That'll buy you a lot more than the marginal increase in CPU. Make sure you shop third-party vendors for the RAM, too. Apple usually has terrible pricing on RAM upgrades.
I would upgrade to the ATI Graphics Card first and more RAM as suggested above.
If you were video editing the performance would be worth it because of encoding, but I don't think you will really notice anything for what you are doing.