I like the sound of Dallas quite a bit, but I've always had a hard time warming up to it as a name. I think it's because I was unhappy when I was living in Dallas.
I known two Dallases, one male and one female. After seeing it on both sexes, I prefer it for girls, but I'll suggest middle names for both.
Dallas Patrick
Dallas Nathaniel
Dallas Jonathan
Dallas Charlotte
Dallas Miranda
Dallas Gillian/Jillian