Based on that, it looks like you made a good choice with the US Airways card, especially for getting to Harrisburg via Charlotte. As for getting to Houston (and international redemption), you should look into http://www.flyertalk.com/forum/miles...nt-credit.html. Spend is only $1000 over 3 months to earn up 50K miles (another 5K for adding an authorized user).

There are other options to get United miles if you're willing to spend a bit more (Sapphire Preferred and certain Ink cards have good Ultimate Rewards points bonuses, which have a 1:1 transfer to United).