When playing games it goes up? Do you have an video card GeForceFX with the vacuum type fan?? The one that takes 2 slots in your PCI. If the answer is yes, then there is your problem. If not, you can invest in a couple of temperature controlled fans for the case like from ANTEC. Look for the low noise level ones. You can grab them from CompUSA.
The other is changing the case. I did that with mine. It decreased 5 degress because the case is alluminium. Decreasing degress means that the CPU fan will spin less, creating less dB
The other option also is changing the PSU. After I changed the Power Supply the noise reduced a lot. THE PSU itself creates a lot of noise.
The problem with the last 2 suggestion is that you have a DELL, and I would not recomend either solution, since it will be difficult or imposible to change the case, and changing the PSU will void your warranty, and when you buy a DELL you pay extra just bcause of the warranty.
Fan?? Your case might have them all installed already, just check they are Temperature controlled. it will have a tiny sensor in the fan