2004 Yukon XL Engine Performance Upgrade

Ok, so in the next month or so, I am going to tear down my motor and replace some leaking gaskets and swap out all the lifters because one has been ticking on and off. What I think I would like to do while I am at it, is to upgrade some of the parts to get some more performance out of the engine.

Basically I am looking for some advice on what to buy specifically to get some additional horsepower, and possibly increase my gas mileage too. Can anyone reccomend a list of what I should use? New heads vs rebuild my heads, new cam, etc... Also, would I need to or should I have the ECM reprogrammed after the changes? Thanks for reading!