Gatsby built himself up to an elite socialite in order to be physically and socially closer to Daisy. For Gatsby, his American Dream, Daisy herself, is tied up with the idea of money. Nick realizes that Gatsby must have accepted this fact. As much of an idealist as Gatsby is, he uses money to get close to Daisy and when they rekindle their relationship, he uses his wealth to impress her.
In Chapter 7, with the rising heat and the rising emotional tension, Daisy asks plaintively, "What'll we do with ourselves this afternoon, and the day after that, and the next thirty years?" Later in the chapter, Gatsby confides in Nick that he thinks Daisy's voice is "full of money." Nick has a revelation:
That was it. I’d never understood before. It was full of money—that was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbals’ song of it. . . . high in a white palace the king’s daughter, the golden girl. . .
Although a romantic idealist, Gatsby had conflated the romanticism of love and money in forming his vision of the American Dream. Daisy represents both of these things for Gatsby: love and money. This speaks to the idea that at certain periods, or perhaps always, the American Dream is relatively or necessarily linked to money. In other words, having money makes it easier to attain such a dream. So, of course having "old money" (inheriting it, being part of the owning class) will make it seem more likely of attaining the American Dream. The irony is that having money does not guarantee happiness. Daisy has money but is unhappy. And Gatsby acquires money but loses his (Daisy) American Dream.
When she says, "What'll we do . . . " Daisy, looking for a reprieve from the heat and tension, is asking what will happen after this. What will happen between herself, Tom, and Gatsby? In terms of "old money" as part of this inquiry, Daisy might be asking with whom will she be more able to sustain her comfortable way of life.