I already have done this, and it seems to be working great! i've become an addict of trying to keep my 4700, which is loaded up with all kinds of goodies, extra cool.

 

I have a standard ball bearing fan that is the exact size of the case shroud's fan on the rear of the case. The one that pulls the air through the green thing onto the CPU.  I rigged it on the outside of the case (twisty ties) pulling in extra air so i pretty much have 2 fans blowing air down inside that green case shroud or whatever it's called.   I also have a pci exhaust fan running on the bottom of my case and i swear its pushing out almost 2x as much air now!  Im getting 10fps more in high line games such as CS2 and Doom3.

 

 

 

2 gig's of ddr2 ram, extra 80gig hard drive, a soundblaster pro audio card, pci exhaust fan,  and of course, a 6600 pci express vid card