Here it says "David v'Chol Yisrael." In Shmuel II, 5:6 it says "David and his men"!
Radak (Shmuel II, 5:6): Now all Yisrael were his men (he ruled over them).
What are Yevus and the Yevusi?
Rashi (Shmuel II, 5:6): Yevusi descend from Avimelech. Metzudas Tziyon is called Yevus.
Vilna Gaon: Yevus is [the name of] their city. Yevusi is [the name of] the nation.
Why did David go to capture the Yevusi now?
'Rashi': It was lest people say that all David's wars in the days of Sha'ul succeeded due to Sha'ul, and [now] he fears to fight. Therefore, he went to war immediately.
Radak (Shmuel II, 5:6): There was a tradition that Tziyon is the head of Malchus Yisrael, and only a Melech over all of Yisrael will conquer it. This was only today; Sha'ul's kingship was not established (permanently).
Malbim (here and Shmuel II, 5:6): Hashem wanted that the capitol city be on the border between the two previous kingdoms (Binyamin and Yehudah). Yerushalayim is listed on the border of both of them (Yehoshua 15:8, 18:16, 28). The Mizbe'ach was mostly in Binyamin, but part was on a strip that extended from Yehudah (Yuma 12a). Yehudah already conquered their part of Yerushalayim; Binyamin had not conquered their part. David went to capture it, to make it the capitol.
