High-performance personal computers have increasingly become thermal nightmares. As GPUs and CPUs become more powerful, their thermal output skyrockets. Intel's Pentium D 840, for example, has a rated thermal output of 130W. AMD's top-of-the-line Athlon 64 X2 4800+ is rated at 105W. GPU heat output is also increasing—and if you're running a dual graphics card setup, multiply that by two. Memory clocks for both main memory and graphics memory are also going up. For example, Samsung just announced 900MHz GDDR3. As hard drive capacities increase, and performance geeks build RAID arrays into their systems, hard drive heat output also becomes a factor. Finally, the power supply itself is getting beefier—which means more heat.
The typical solution to the problem is to throw extra cooling at the system. In other words, add more fans. Pretty soon, you've more fans in your system than the old B-36 bomber had propellers—and your system is just about as loud, too..