I've already picked out the CD I plan to buy next.
Literal
Next [to-に] buy [intention-つもり] [attributive-の] CD [topic-は], already chosen-[resultative-てある] [explanatory-んだ].
つもりの is the attributive form — 'the CD that I intend to buy.' ~てある indicates the choosing is complete with a lasting result. もう reinforces completion. んだ adds explanatory nuance. Subject omitted throughout.