You need to reach 2,500 posts to have an underlined name.
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I'M JESUS (09-13-2011)
Member level 1 (0-2499 posts) = 150x175 avatar and nothing
Member level 2 (2500-4999 posts) = 175x200 avatar and underlined name
Member level 3 (5000 - 9999 posts) = 200x225 avatar and bold name
Member level 4 (10000+ posts) = 200x250 avatar and bold name