CLASSICAL/OPERA. Henry Purcell - Dido and Aeneas (Philips): John. Eliot Gardiner's tempi and orchestral balance in this new ``Dido and Aeneas'' make the opera flow naturally and with great dignity. This is not a collection of isolated vocal pieces, but a unified conception. The Monteverdi Choir provides much of the beautiful music; the famous echo chorus is exquisitely sung, as in the emotional couplet ``Great minds against themselves conspire/ And shun the cure they most desire.'' The soloists all perform with appropriate simplicity. Ruth Holton's Belinda stands out for purity of tone, Carolyn Watkinson's Dido for her plaintive yet regal approach. With generally brisk tempi and only two short instrumental pieces interpolated into the score, this ``Dido'' is over quickly. Philips has filled out the disc with a recording of the 1638 ``Ode for St. Cecilia's Day,'' which balances well with the opera.