As you have asked this question you may be knowing that Vedas are the most ancient available literature on earth. Rig Ved, YajurVed, Atharva Ved & Saam Ved are the four collections among which Saam Ved is not read, it is sung. Persons who know the Vedas are very few. but you may search on the net you will find the recitation of Saam somewhere. So obviously Indian classical music has roots in Saama Ved but it is also true that over the centuries various invaders who came to India brought their cultural effects in the Indian classical and it got enriched. The Indian classical we hear today is much different than the ancient one and that ancient music is very rare to find in pure form, leave aside your question if a song can be as per Saam because Saama Ved is a literature which is sung as per the ancient singing tradition. Now next question What is a Raag? I will try to answer this as my mother is PG in Indian classical music. A Raag is a set of musical notes which broadly defines the frame of a musical performance be it a Khyaal, Thumari, Bhajan, NaaTya geet, Film song etc. some notes are sung, played predominantly while some are not at all. But beauty of Indian Classical is that every time a performer performs it is a new creation although he / she is honoring the Raag framework. feel free to ask questions I will try to answer as I find time.

Thank you Tushar

the question is difficut and unanswerable for anyone not from a classical musical background, and i am not the one :D. here, there is some sensibe informaion regarding this. http://www.sushmajee.com/reldictionary/dictionary/page-S/sangeet.htm ...Also u can look at what guy has to say about development of the swaras and samaveda music here.. http://creative.sulekha.com/music-in-sama-veda_362456_blog .(see section 7 if article lookss too long) Some points here, may be their own views on which some might differ.