OK, San Pedro de Atacama is awesome too, I loved it, and it’s always full of tourist so is really fun. Kindda expensive though, but there’s good food. The Salar de Atacama is beautiful, I hope you’re lucky enough too see the flamingos flying (I saw them!!). I do not have too many tips cause I went there just last year in a school trip…so I couldn’t wander around; but the trip worth it, everything is breathtaking. Oh yeah, go to the museums in San Pedro and Arica, they’re good.
The south is AMAZING! Pucon is my favourite place to go on summers, though is crowded in February, luckily you’ll skip that. There’s lots of things to do!, canopy, hikking, even paintball; the lake and the volcano are beautiful (I don’t recommend you Pucon’s main beach, is disgusting, you better find a more private one, there’s tons in the way to Villarica). The city is really beautiful cause they planned to build it in wood with a German style..oh yeah, food is good too, be shure to try German cakes and pastries.
I haven’t go to Chiloe (in fact is in my 43 places I wanna go), but I know is incredible since most people I know have seen it, so you’ll have a great time there.
Osorno…my cousins lived there so I went there like 10 times or more…the city it self is not very appealing, kindda boring in fact, but the sorroundings are sooo gorgeous, the Volcan Osorno is one of the most beautiful of the south, and during this time of everything is green and nice. Near Osorno there are various hot springs like Puyehue Resort (you can go for a day), or the Lake Ranco (for me the greater one in the south). So if you want to go to Osorno I recommend you to trip around the city. There are great German restorants in there too.
I haven’t go to Puerto Montt either, so I can’t tell you anything about it :P
You can pass to Argentina from Osorno, in bus through the Cardenal Samoré way, or also in bus from Puerto Varas through the Parque Puyehue pass, or from Puerto Varas to Bariloche in a boat, the so called “Cruce de Lagos” http://www.visitchile.cl/crucedelagos.htm

Oh yeah, some tips…Chile is really safe, but be careful nevertheless, since “gringos” are sometimes appealing (if you’re in touristic places I don’t think you’ll have problems though). Things are way cheaper than in the States, but have common sense; is better to buy things were chileans buy things. Some flee markets are really cool, Santiago has some good ones, and San Pedro too (but is expensier there). Food is good, not as “exciting” as mexican, the best thing in Chile are the basic stuff; fruits are the best, and you’ll be lucky to come here during summer! Try rare ones like chirimoyas or lucumas, and in the south berries are good. I really recommend you German pastries in Pucon and Osorno!

I hope this helps! You’ll love Chile :D have a GREAT trip.