Depends much on what you're looking for. If you're just looking for cash back on the regular travel you already do, not really my cup of tea but I would think you can just keep a United and Delta account, or whomever you fly with in the future. With only a few trips a year, focus on one isn't going to get you much status, but you might get low level and free bags.

If you want to get into it more for using those miles, are you looking for biz class or coach? Domestic or Int'l? Single or family of 5? Those kind of things will help you decide on what programs to join and how to handle it.

If you're willing to get in to the hobby in more depth, read the info here and on the blogs. But beware the blogs as they will not give you all the bad side of cards and they will pimp whatever card gives them the most commission. So always compare blogs with flyertalk with friends opinions etc., and buyer beware.

Anyway, long story short, I'd go with United, Alaska, Delta and American to start. Maybe Southwest too if you fly them at all. And sign up for awardwallet or something similar to keep track of your miles (although I have to manually input some accounts on there nowadays). I would not reactivate US airways, you can buy their miles for super cheap sometimes anyway.