Power problems with Dell Dimension 4600

My computer is not starting up at all. After some online research I discovered this model has had problems with faulty power supplies. So I replaced the 250 W with a 450 W. Still the same response, nothing except the flea power light on the mother board. Not even the fan on the new power supply is running nor the CPU fan. Different configurations of wiring from the power supply have all yielded the same results. The capacitors all look fine, no leaks or bulges. One thing though is that the paste/glue holding down the heat sink has deteriorated so that the heat sink is no longer seated and comes off. Would this affect start up power? Would replacing the paste make a difference? Do I have a bad CPU or motherboard?