I always go through an upgrade sequence: components, positioning, interconnects and cables, power cords, enjoy for seven years of so.
The effect of wires can vary pretty widely. But I guess, in general, I'll get 80% of the sonic improvement from component changes... then 10% from wires and another 10% from positioning and acoustical treatments.
You have to balance the likelyhood that you will do some component upgrades. If you think this is likely, do these first... then follow with wires. You want the wires optimized for the components you own.