Next stop, South Beach. The Knicks extended their winning streak to a season-high eight games last night with an early 3-point barrage and went on to a 108-89 victory over the Celtics at the Garden. They will carry the league’s longest current win streak into Miami for tomorrow’s showdown against the defending champions, whose 27-game win streak was snapped last week. The Knicks took the season series against the Celtics, 3-1, and can capture the season series against the Heat by the same count with a triumph tomorrow. They have won two of their three meetings with Miami, but those wins came in November and December. “Anytime you can take three out of four wins against a team like Miami or Boston, that’s always good,’’ Carmelo Anthony said. The Knicks shot 11 of 16 from 3-point land in the first half to highlight the rout. Anthony finished with 24 points and 10 rebounds. J.R. Smith added 15 points off the bench with 12 rebounds and four assists, as his streak of three straight 30-plus games ended. Raymond Felton had 18 points, including a half-court, halftime buzzer-beating heave that electrified the Garden, and rookie Chris Copelean added 22 for good measure. This marks the first time the Knicks took the season series from Boston since 2003-04, and it was their second blowout win over the Celtics in five days. The Knicks and Celtics could meet in the first round of the playoffs as the two-seven matchup. The Knicks (46-26) are tied with the Pacers for second in the East nd lead the Nets by 4 1/2 games in the Atlantic.