tiny voices is right. Mount the PSU so that it will pull in cool air from outside the bottom of the case and then vent it out the back. It will also prevent the PSU from discharging warmed air into your case, because discharging air into the case from the PSU just puts warmer air inside your case. Don't start out with pre-warmed air for your heat sink fins to have to battle. That cuts down on how effective your heat sink will be.
Also, make sure you mount a fan on the front, somewhere between the front panel and hard drive bays, that will pull outside air into the case (as an intake fan). The more your case can keep cool air in, the cooler your components will be. Then, your fan on the back should be oriented to push air out the back of the case (as an exhaust fan). The more you can keep the hot air out of the case, the cooler your components will be. Yes, I am pretty much repeating myself, because it bears repeating.
Your CPU setup should be fine, as long as the direction of airflow from the fan on the heat sink is aimed at the back fan for exhaust out of the box as quickly as possible.
The problem with heat sink/fan combinations is that while they do cool the CPU, they radiate all the heat from the CPU directly to the inside of your case. That is why adequate airflow through your case becomes paramount. Front fan to bring cool air in, back fan to take hot air out. That's a minimum requirement.
Later, when you can afford more fans, put another intake fan on the bottom of your case, and exhaust fans on the top of the case. That will help keep it even cooler. With more fans, you can lower the RPM of each fan and get a quieter case overall.