In what chapter does Elizabeth Bennet visit the Pemberly Estate? This is when she thinks it is so beautiful and says something like "and I could have been the mistress of all this" I think.
That would be Chapter 1, Volume III (Ch 43):
"They descended the hill, crossed the bridge, and drove to the door; and, while examining the nearer aspect of the house, all her apprehensions of meeting its owner returned. She dreaded lest the chambermaid had been mistaken. On applying to see the place, they were admitted into the hall; and Elizabeth, as they waited for the housekeeper, had leisure to wonder at her being where she was.
The housekeeper came; a respectable-looking, elderly woman, much less fine, and more civil, than she had any notion of finding her. They followed her into the dining-parlour. It was a large, well-proportioned room, handsomely fitted up. Elizabeth, after slightly surveying it, went to a window to enjoy its prospect. The hill, crowned with wood, from which they had descended, receiving increased abruptness from the distance, was a beautiful object. Every disposition of the ground was good; and she looked on the whole scene -- the river, the trees scattered on its banks, and the winding of the valley, as far as she could trace it -- with delight. As they passed into other rooms, these objects were taking different positions; but from every window there were beauties to be seen. The rooms were lofty and handsome, and their furniture suitable to the fortune of their proprietor; but Elizabeth saw, with admiration of his taste, that it was neither gaudy nor uselessly fine; with less of splendor, and more real elegance, than the furniture of Rosings.
And of this place,'' thought she, I might have been mistress! With these rooms I might now have been familiarly acquainted!"