Specs needed for Photoshop to fly.

I'm upgrading from a late 2008 iMac with a 2.4 Intel Core 2 Duo and 4Gb of Ram to a new Retina Macbook Pro 15".

Will processing power for example (2.3 - 2.6Ghz) make a big difference in the way Photoshop runs? What about Ram? What else is needed for Photoshop to handle big files and still run smoothly?