Dallas is a great city, and you'll probably enjoy it when you are there! There is a huge nightlife scene, but you have to know where to go, since everything is so spread out. The style is preppy casual, and that's so easy to do in a hot climate. I would go for some nice short-sleeved dress shirts (yes, they wear them there), longer shorts, and very light khakis. You could even get away with some nice leather slip ons or sandals, probably. And even if you aren't a hat guy, you'll want a baseball hat to keep on standby, because hats actually help a lot with sun, and if you are at any outdoor activity (drinks, concert, etc) you might find yourself wanting a hat.
Good luck and have fun!