The Noble Qur'an Al-A'raaf 7:26
O Children of Adam! We have bestowed raiment* upon you to cover yourselves (screen your private parts, etc.) and as an adornment, and the raiment of righteousness, that is better. Such are among the Ayât (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allâh, that they may remember (i.e. leave falsehood and follow truth).
The Noble Qur'an 7:3 [translated] [Say (O Muhammad SAW) to these idolaters (pagan Arabs) of your folk:] Follow what has been sent down unto you from your Lord (the Qur'ân and Prophet Muhammad's Sunnah), and follow not any Auliyâ' (protectors and helpers, etc. who order you to associate partners in worship with Allâh), besides Him (Allâh). Little do you remember!
Narration - al-Hakim and Tirmidhi (hasan) "Allah likes to see the mark of His benevolence on His servant." Wearing the izar (loin-wrap), thawb (long cloth), `imama (turban), jubba (coat or mantle), sirwal (baggy pants), etc. is of the Prophet's (SAAWS) Sunnah of dress. There is no such thing as an unimportant or unnecessary Sunnah.
Hadith - Tirmidhi (hasan), Book of knowledge; al-Baghawi, Sharh al-sunna 1:233. Whoever gives life to one of my Sunnahs which was eliminated after my time will receive the reward of all those who practice it without their reward being diminished...
Hadith - Tirmidhi (hasan gharib), Book of knowledge. Whoever gives life to one of my Sunnahs, he loves me: and whoever loves me is with me.
And Abu Hurayra (r.a.) narrates the following hadith in Muslim, Nasa'i, Malik, and Ahmad:
The Prophet came to the graveyard and said: "Peace be upon you, O abode of a people of believers! We shall certainly join you, if Allah will. How I long to see my brothers!" They said: "O Messenger of Allah, are we not your brothers?" He replied: "You are my Companions! As for my brothers, they are those who have not yet appeared." They said: "How will you recognize those of your Community who had not yet appeared (in your time), O Messenger of Allah?" He replied: "Suppose a man had horses with shiny white marks on their foreheads and legs: would he not recognize them among other horses which are all black?" They said: "Yes, O Messenger of Allah!" He continued: "Verily, they (my brothers) shall be coming with shiny bright foreheads and limbs due to their ablutions, and I shall precede them to my Pond."
Dressing according to the sunnah is an act of ibadah that brings with it a reward, insha'Allah.
The very least that has been said by the scholars of the Shari`a in the matter is that following the Prophet in matters of dress or everyday matters such as eating, walking, and sleeping is a matter of excellence (ihsan) and perfection (kamal) and is desirable (mustahabb) and part of one's good manners (adab) in the religion. Every desirable practice performed on the basis of such intention means a higher degree in paradise which the person who neglects it may not attain to, and Allah knows best.
Modesty for Men: http://www.suite101.com/print_article.cfm/muslim_women_retired/61371
Comprehensive Guide to Muslim Dress Code for Men and Women: http://www.modernmuslima.com/hijabhow.htm