I'm with redoXan, if you power supply is already blowing out, then you need to force room temp. air in with the second fan. I cut a hole in the bottom of my case(3X5)and put a filter over it. An 80mm fan is installed directly over the CPU & Swiftech heatsink (screwed to the side panel)with a duct forcing the air though the sink. The sink gets a constant rush of room temp. air!!! The PSU has two exaust fans & there's one more 60mm blowing out the rear of the case. My temps for my 2.4B @ 166FSB (3.014GHz) runs in the mid 50's under full load & high 30's @ idle.
My PSU is an Antec 340w (came as a part of my case, a Chieftec/Antec Dragon series - very good BTW), and has one fan to blow air out the back. It seems to work well enough as it is, and has even coped with my adding acoustic material above and beneath it without heating up measurably.
Then again, i'm not in front of that computer and might be talking ball-cocks, but from memory that's right.
I'll confirm this later the afternoon.
But that isn't completely true... a lot of people buy cheap PSUs which only have one fan and even high-end PSUs have single fan editions. And I have a fairly new Codegen 350W as second PSU and it almost hasn't any grills.
My CPU fan spins so fast that it creates a wormhole