So, when Cataclysm drops, Tier 11 raids will be current content, and the heroic 5-mans will be "previous content". So your JP will buy gear that is sufficient to get you into raids.
From being on the beta, I know that questing and dungeons will end up replacing much of your gear by the time that you hit 85. Some of that gear won't be more than level 300 gear, meaning that you may have to flesh out your armor (so to speak). For instance, I need to buy a helm and some gloves to replace some early greens.
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