For admission to a selective school, how much weight in terms of percentage is put on the ACT/SAT (+subject tests) vs. the GPA? I mean ideally 4.0/36/1600 but how is that looked at? 4.0 GPA and crappy test scores vs. poor GPA (3.0-3.3 or so) and stellar scores?

It really depends on what school you're looking at, I think. Some school that I've looked at have it sorta like 25% for each factor but I only know a few schools that do that and then you have others that figure in other things you do... volunteering etc. Check the school websites that you want to go to and they will tell you exactly.