Maria - I haven't been to those Northern Chile Nat Parks but I have been to relevant parts of Boliva, right next door and even higher altitudes. My son-in-law visited the area you're interested in only a few months ago and came back raving about it.

Re the altitude issue you will find a detailed article at http://www.princeton.edu/~oa/safety/altitude.html which I found very useful. I was desperate to get into the high Andes but also was worried about altitude sickness as I've had a couple of heart operations. In the event I was quite OK at and above 4000m but I did consult doctors first and take precautions along the lines of those that you'll find in the article.

In my post (Part 1, I still have to submit Part 2) to TheTravelzine about the Bolivian trip I made some comments about altitude - http://groups.yahoo.com/group/TheTravelzine/message/40131

Have a good trip Michael Sydney, Australia