If you are using a factory harness and are after an easy ECU upgrade, take a look at Haltech Plug 'n' Play Adapter Kits. These adapters plug directly into your Haltech ECU and have a set of factory connectors on the other side which plug straight into the factory harness.
If you are after a simple plug-and-play option and your car still has its original engine and wiring in it, a Plug-in ECU is the easiest and quickest solution.