Replacing the fan or fans in a dell 4600 with the quietest fan I

Identify each of the fans in the chassis in terms of its square size and thickness, is it 3 wire or 2 wire, confirm its voltage as 12 volt and what current it draws if it tells you on the label. Unfortunately due to the nature of Dell chassis design it its normally difficult to fit anything other than the standard size in the space provided and they mostly use either 80mm or 120mm fans. You than need to look for replacements with the highest airflow in cubic feet per minute (cfm) versus noise (dB). Zalman are normally credited with producing the quietest fans and if you can replace a 80mm with a 120 then you could use a separate speed controller to reduce the speed of the 120mm fan to a slower revs as they by default have a higher cfm (bigger area). This is where the current comes in as 120mm fans generally draw a higher current at full speed but you can normally get away with 1.5 times the stock fan current.

Beyond this requires some serious case mods but the most critical is the CPU fan. If the PSU is the problem them some Dell cases will allow you to fit standard models with bigger temperature controlled fans.

First you need to tell us which fans are the loud ones you are talking about but normally it is the cpu fan. Any computer parts store should have various fans available for different chipsets.
This link will take you to a page to find a fan. You will probably have to get the model number off your current one to find an exact match

http://www.frozencpu.com/cat/l3/g36/c15/s700/list/p1/Fans-12_Volt_Fans-92mm_x_38mm_Fans-Page1.html