The lump between Chile and Argentina

Hi,
I am planning on cycling around South America, starting in Santiago at the end of August.
The first part, which I would like to start planning, is getting over the Andes and into Argentina. With it been winter still in August I m thinking I might have to hang around in Chile for a wee while before going up into the "hills" or head North. I'm keen to go to Buenos Aires so don't want to head to far north just to head south again.
Has anyone got any recommendations on where to cross, and how soon in the Spring it is doable to do so!?
Obviously keen to avoid the busier roads....
Any other general advice for cycling in SA would be great too!
Cheers
Richard