If you have really crappy cooling right now it will make a big differance. Normally with good cooling you should run 2-3'C. Even 3-5'C warmer is OK. If your room temp is 22'C and you run 40'C full load, then a side and top fan could drop your temps 10'C or more, but so could cleaning out the dust, tie loose cables out of the way of the airflow and removing restrictive fan grills and installing wire grills (like used in most blowholes). If you have one fan front and back (not counting the power supply fans), you should be able to get at least adiquate temps. If not this will drive up the temps of all the parts in your computer including memory, CPU, graphics card, hard drive, CD... and some of these componants can be pretty sensitive to high temps, causing stability issues or even early failures.
My case has 2 fans in front, 1 in the back, 2 in the power supply and one on the CPU. These are all low RPM (2500) 80mm fans that are very quiet (30db or less). I have a large passive heatsink on the northbridge and the stock heatsink on my GeForce4 Ti 4200 (whick is pretty quiet). My case temps are usually about 3'C above room temp full load and the passive heatsink on the northbridge stays about 30'C. Some said these heatsinks kept the northbridge as cool as the stock heatsink and fan, but it actually runs about 5'C warmer, but at least I do not have a fan that keeps failing on me (I had to clean and oil the cheap thing about every 3 months to keep it from grinding or stopping). My heating is an old GlobalWin FOP32-1 that I mod'd to fit an 80mm fan (spred the fins out a bit). I use Arctic Silver II and the fan generates 34CFM at 30db (2500RPM) and my system stays at 32'C idle (with CPUCool running) and about 42'C full load (almost the same as my northbridge). My room temp is 22 and my case temp is 24'C.
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