I really like the synergy you have going with Sun Titan and your creatures & Oblivion Ring. Is the blue really worth it though? Geist is a great card, actually, and I am by no means saying to make it mono-white. Not at all. I do think that maybe running three is not a great idea. It would be that one extra card in your hand when you can easily get it back with Sun Titan from the graveyard. But this is such a nitpicky point, and I'm probably wrong--you don't get to know these nuances until you've played the deck a bajillion times, and you know best. This deck looks epic, though. I wish you all the best in getting those Swords.
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I'm going to have to voice another objection to the Phyrexia's Core & Tumble Magnet combo. It's a great combo, and you're right that they fill multiple purposes. But tapping Phyrexia's Core instead of using it as mana just to gain one life, I don't think, is worth it. Also, Sun Titan's ability should be reserved for more important things, like getting the Snapcaster Mage back. I doubt that you will at any point choose Tumble Magnet over Snapcaster Mage. Of course, I've never been very versed in the mysterious ways of blue, so I may be completely wrong. If it's that you fear the deck will be so behind that life gain is worth it, white has way more powerful lifegain than Phyrexia's Core.