Dang. Somebody already suggested the Ted Chiang story. But consider it seconded.
As DM, assume that if the fight is a planned fight, then everything goes his way. He has whatever spell he needs memorized and available as necessary. Have him wear a ring of epic spell storing, and have it contain, again, whatever spell he needs. With whatever metamagics he needs. He's that intelligent. He planned everything.
Also, look up feats or abilities based on intelligence. A level of, what, Duelist, for Int-to-AC? Arcane Strike (he guessed vaguely) for Int-to-attacks? Is there anything that lets you add Int to saves, like a Paladin adds Cha? There oughta be. That would be sweet.
Roleplayingwise, you've got a few options:
-Brooding and quiet: Others have suggested this. Good option.
-That ghost-guy from Angel. He sounds like a dope a lot of the time. He tosses so much garbage data out there that you take him for granted, never realizing that that's exactly what he wants you to do -- that he had the whole thing planned from the get-go, and that you've just done exactly what he wanted.
-Crazy. Always fun to be crazy. I mean, he can take out an 18th-level party with Grease, just because of the save DC. He can play "How many magic missiles will it take to kill you" all afternoon. And so on.