Two and half weeks should be plenty, although if you want to travel overland, you will have to battle through a few long bus journeys. As long as you're careful to book decent buses with good reclineable seats you should be fine though. We did this route in November 06 over the space of about 2 weeks and it was fine.
We took a bus all the way from Cusco to the Peru border at Tacna (overnight - about 16 hours - make sure you take a blanket or sleeping bag!) - from there you can get across the border to Arica, (if you're lucky, in an old school open-top Cadillac!).
Not sure exactly what's in Arica as we didn't hang about - perhaps someone else can advise. From Arica we took a four hour bus direct to Iqueque, a fantastic seaside resort on the north coast. We stayed in a great hostel called 'Backpackers' - there's a few things in the area to see too. We hung around for a week as it was so relaxing after all the trekking and partying in Cusco.
From there we took a direct bus to Santiago (24 hours) but you could perhaps think about breaking this up with a stop inland at San Pedro de Atacama. We stopped here on our way up from Argentina, and although it's a small town, there are plenty of trips to do in the area such as Valle de la Luna and the Geysers del Tatio. Also, if you think you can squeeze in a four day trip across the Bolivian Altiplano and over the Uyuni Salt Flats I would highly recommend it - by far one the best highlights of our South American trip!
Personally, I think you can get away with only spending two or three days in Santiago as there are much more interesting things to see elsewhere.
