The Shulhan Aruch (Orah Haim 175) codifies the Halacha requiring one to recite the Beracha of "Ha’tob Ve’ha’metib" when he drinks two kinds of wine. This Halacha applies when a person drinks one kind of wine, and then afterwards drinks a second wine of the same or a higher quality. So long as the second wine is not inferior to the first, one recites this Beracha.
Elsewhere (see Halacha entitled "Reciting the Beracha of Ha’tob Ve’metib Over a new Bottle of Wine" dated Nov. 12, 2007), we enumerated the various conditions that must be met for this Halacha to apply. Here, we will touch upon two other issues addressed by the Halachic authorities. Firstly, the Mishna Berura (Rabbi Yisrael Meir Kagan, 1839-1933) writes (175:5,14) that one recites this Beracha even if the second wine was present on the table when he recited the Beracha over the first wine. We might have considered limiting this requirement to a case where a person drank some wine, and then a different wine was brought to the table. In truth, however, one recites this Beracha even though both wines were on the table initially, and even if the individual knew when he drank the first wine that he would later drink the second wine. This is also the ruling of Hacham David Yosef, in his work Halacha Berura (vol. 9, p. 132). He cites those who advise keeping the second wine off the table while reciting the Beracha over the first wine, so that one can recite the Beracha according to all views, but according to the accepted Halacha this is not necessary. One recites "Ha’tob Ve’ha’metib" before drinking the second wine even if it was on the table when one drank the first wine.
Secondly, the authorities discuss the question of how much wine one must drink for this requirement to apply. Some authorities maintained that one recites "Ha’tob Ve’ha’metib" only if he drinks a Rebi’it of the second wine. However, Hacham David (ibid. p. 140) rules that the quantity of wine that one drinks has no bearing on this Halacha. Even if a person drinks only a sip from two different wines, he recites "Ha’tob Ve’ha’metib" before drinking the second wine. The reason for this ruling relates to a different Halachic issue, namely, the fact that the Beracha over wine ("Boreh Peri Ha’gefen") covers all beverages that one drinks subsequently. Halacha follows the view that this applies regardless of how much wine a person drinks. Even if a person drinks only a sip of wine, the Beracha he recited before drinking that sip covers all beverages he drinks thereafter. This demonstrates that Halacha affords significance to even drinking small amounts of wine. Hence, with regard to "Ha’tob Ve’ha’metib," too, even a small sip is significant enough to warrant the recitation of this Beracha.
Summary: One who drinks wine and then drinks another wine of at least the same quality recites the Beracha of "Ha’tob Ve’ha’metib" before drinking the second wine. This applies even if the second wine was on the table when the person drank the first wine, and regardless of how much or how little wine he drinks.