Which comes first, the cleaned-up park or the prospering community? In the case of this vibrant slice of ever-transforming Harlem, it’s a little bit of both. Marcus Garvey Park itself is undergoing a near–$8 million renovation, including smoothed-out sidewalks and the transformation of its existing bandshell into a state-of-the-art amphitheater. Amid the neat stretches of always-in-demand brownstones, a 28-story glass condo with an indoor pool (5th on the Park) has sprung up on Fifth Avenue and 120th Street, while nearby, on 123rd Street, there’s the recently constructed Windows on 123, a pair of buildings graced with limestone-balconied apartments and full-floor lofts.