It really depends on how "smart" your PC is. Some will shut down if they don't detect proper fan speed (3 wire fan). Others shut down on a CPU thermal out of spec. Others just crash Windoze and go to sleep. The CPU activity drops to zero and reduces the heat load.
A general rule is you can't have too big a heat sink and fan ... as long as it still fits. Back in the olden "pentium 1" days they might not even use a CPU fan, just a giant heat sink. Now you get a giant heat sink and a fan or two.
