A 484b

We're sorry, the rules text you're looking for doesn't exist. I've never seen an adequate response to this concept in RAW. However, the PHB1 FAQ does address the topic unequivocally:
Can a 5th level wizard memorize two 5th level daily spells instead of a 1st and a 5th level daily spell?
At the end of each long rest, a wizard prepares one 1st-level Daily spell chosen from the list of 1st level daily spells in her spellbook. If she were at least 2nd level, she would also prepare one 2nd-level utility spell chosen from the list of 2nd-level utility spells in her spellbook. At 5th level she would also prepare one 5th-level daily spell from her list of 5th-level daily spells in her spellbook. She would repeat this process for every level that she has access to wizard daily or utility spells. She could not, as a 5th-level wizard, prepare a second 5th-level daily spell in place of a daily spell of a different level.