>>Up in Hinterlands there is a bunch of elves that are alliance friendly. Did they used to have the nelf model and now have the belf model? Are they high elves? Why are they friendly to Alliance?

Yes, those are high elves, and no, they used their own, not-very-pretty model before getting replaced by the shinier blood elf model, since they're one in the same. The blood elves are simply high elves who followed Kael'thas Sunstrider and rely on fel magics as substitute to the Sunwell's energy. This addiction leads to their green eyes, in contrast to the standard high elf blue. The high elves you see in Eastern Plaguelands, the Hinterlands, Dustwallow Marsh, and other locations have the proper blue eyes. Why are they allied with the Alliance? The high elves were part of the Alliance of Lordaeron during the Second War (Warcraft 2). Although their alliance would weaken by the time of the Third War and the fall of Quel'Thalas (Warcraft 3), many high elves were still part of the Alliance forces. If you play the Human campaign, you'd notice that the priests and sorceresses are high elves. Because so many high elves perished during the Scourge Invasion and so many of the survivors followed Kael, high elves seem to be the minority now, but those that were with the Alliance are most-likely still with their human allies. For more information on the differences between high elves and blood elves, and where you can find high elves in WoW, check out their articles on WoWWiki:
http://www.wowwiki.com/High_elf
http://www.wowwiki.com/Blood_elf

Back to your original question, I'll leave it that it would be highly unlikely for the Naga to join any side of the conflict. They're a nasty bunch with their own agenda. For information on that, I'll direct you once more to WoWWiki:
http://www.wowwiki.com/Naga

And, of course, a lot of this information can be found by playing (or cheating) through Warcraft 3 and the expansion, the Frozen Throne (especially relevant for the Burning Crusade and Wrath of the Lich King, including an answer Travis's question about Dimensional Gateways). The game is an incredibly fun time, and this is coming from someone who doesn't usually enjoy RTS games. It has a lot of RPG elements that will make a WoW player feel at home. For all of your other needs, WoWWiki is a limitless resource for Warcraft information! Give it a try, there's little you can't find there.