In reply to flett53: I'm not sure if you need something special for altitude (I guess you might not want to be blowing much up if you are out of breath?) but can tell you what I find works for cold temperatures.
I use a foam mat with a 3/4 length ultralite (or whatever the thin ones are called these days) thermarest on top. For about 15 years I've used the same Ridgerest but last year went for a Z-rest just because it packs a bit neater. This works well for Scandinavian winter conditions down to the coldest I've been willing to sleep out - around -28). You then have two mats that will work separately for summer camping, together they give loads of insulation and even if the thermarest punctures (mine has twice in 20 years of reg. use) the foam mat will still keep you warm-ish.
