In patch 1.0.5 :
• Affixes on items will now roll their level based on the level of the monster killed rather than the item's level (this applies to all items including random affixes on Legendary and set items)

Act 1 monster mostly are Lv 61 .
Act 3 monster mostly are Lv 63 .

Farming on act 3 have better chance to get great affix items .

This is Act 1 monster list = http://db.d3guyz.com/monster/view?m_act=1&page=1