I just bought a bunch of new parts for my new build, but its my first. I have a big ugly full tower Antec case, that has places for 2 80mm fans in the rear and 2 80mm fans in the front. Do I need to put 4 fans in? My new system is:
Athlon 64 3500
Asus A8N5x Mobo
Geforce 7900 GT
1GB Corsair Valueselect
FSP 400W PSU
1 250GB 7200RPM HD
A couple optical drives
Sound Blaster Audigy Soundcard
I don't really plan on overclocking, and will probably stick with the stock coolers on the cpu and mobo for now.
I presonally prefer 120mm fans as they are quieter and move the same amount of air. You will see cooler temps with all 4 fans but you could also mod the case and add a 120mm fan to the side panel as intake to provide cool air right over the cpu fan, and add a 120mm fan at the top of the case to help exhaust hot air. If you do that then one 80mm fan in front and at the back would be ok.
That machine won't get hot at all. You could put 2 fans in the back if you want, but one will probably be fine.
One thing I do, however, is if I only have one fan in the back and there are 2 spaces for fans, I cover over the spare slot with packing tape. You want the air coming in from anywhere other than the spare slot on back. I sometimes also cover the slots on the side of the case with tape too, but I don't recommend you do that unless you sort of have an idea how air is moving through the case and if you need to so it.