I'm not sure if Jack thought about dressing up to meet Ennis. I think Jake's wardrobe in this scene means to show what Jack was willing to give up. He was definitely getting used to a life where he had everything he wanted, yet he would just throw all that away, and go back to being poor as long as he was with Ennis. I know he had plans to open a ranch, but not all ranches make money. Also, it will probably be a few years before they actually begin profiting from a ranch, if they ever bought one.
If you really look at this scene, there's a strong contrast in it. Jack driving rather fast into the driveway, pulling off at once, happy and joyful, willing to start a new life right there. And as the rich man he is, he's wearing expensive clothes, a nice texan jacket, a good hat and is driving a new truck. However, all you see around him is an old house, a beat up truck, and a man dressed in old clothes. I think that's what the fancy clothes means: Jack's will to give it all up for Ennis.