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It's still there. Just pick base quad-core, then upgrade the configuration to a 6-core. I believe this is how it was always listed; it's what I remember, anyway.

Yes, it was always that way. It took a while for me to find that out since I was drooling over that configuration when it was first announced. The quad and hexa cores are single chip configs so they'll be in the same listing.
 
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