If I might ask why are you 'upgrading' to iOS 4? I think all you get with the 3G is the unified email inbox and folders.

On the downside...well....even on apple's own forum the the views aren't positive

Apple - Support - Discussions - iPhone 3G running iOS 4.Lag ...

Apple - Support - Discussions - 3G upgrade to 4.0: how do I downgrade back to 3.x?