It's also really important to note that Rampage only triggers when hitting with a barbarian attack power, while Swift Charge triggers regardless of the power that downed an enemy.
For the sake of drama and fun, we rule that Rampage and Swift Charge can take place in any order.
Also worth mentioning here is the feat Charging Rampage in Paragon.
You can turn the Rampage free attack into a charge, which means you can use Howling Strike and all the goodies that optimize a charge attack. The chance to hit and dish out tons of damage will increase a lot, particularly with combos where you crit on 19 and 20, like Jagged weapon, or Draeven Marauder.
With attack powers like Storm of Blows (CON = # of attacks), Rain of Blows I tend to go the "crit fisher" way now (jagged), rather than damage output (bloodclaw, reckless).
The extra attacks from critting are pure joy in the game, and now you can even charge when it happens? Can't wait for the game to try it out.