The way I see it, Alistar's main function is repositioning the most dangerous (and squishy) members of the opposing team. He needs to get in, knockup, headbutt, get out. And live. And repeat.

Repeat, live = CDR, durability. Hence, Spriit Visage- magic resist (spells > ad early game) and CDR. Next, Glacial Shroud- armor, CDR. With masteries and runes, your CDR is pretty much maxed already, and you have near 100 in armor and MR, and you probably get to buy at least one more big item still. (I recommend rylai's for hp - third in the trifecta of tank stats - and slow. Or, buy more MR or Armor depending on what you're up against.)

This leaves one detail unexplained- early game, you're not about damage, you're about keeping you and your lanemates' hp up and repositioning. The damage from headbutt and pulverize comes from your tower. Take headbutt first- if you're good, you can flash behind an enemy and knock them into your tower at level 2.

You do not want to rum out of mana and miss the opportunity. Hence, chalice. Also, it's early MR.

Give it a shot.