From what I understand there will be a slight performance boost, nothing to write home about. Whether or not it is worth it will depend on how much you value the other features.
The A8N32-SLI has an extra gigabit network card, Built in Firewire support, Better power management (It is supposed to draw less wattage, I don't know if it does in practice), It has external SATA hardrive support (Outside the case) not to mention it uses fanless cooling.
So if these things are worth the price of upgrading to you. Go for it. Other wise skip it.