Power cords. As I've found recently, every other change/upgrade you make is affected by your choice of AC components. Dedicated lines first, if possible, and then power cables, interconnects, speaker cables.
I'd also avoid committing alot of money to power conditioners early on, as they usually make hearing component differences even more difficult. A simple, but fairly transparent device like a Balanced Power Tech CPC, or Shunyata Hydra would be my choice.
JMO