The Readme file for SQL Server 2005 says how much space will be used on the system drive, and how much will be used for the program files if you direct the install to another disk. The Readme files for the SP upgrades have similar information for the additional space they require. While a SP is being installed, you normally need about 0.5 GB work space on the system drive, and a further 0.6 GB space in the system /Temp folder.
It is likely that after 3 to 5 years of applying maintenance to SQL Server 2005 you will use about 2.5 GB on the system drive, and about 0.5 GB on the program file drive. You need to plan the size of your system drive to allow for this.
The values for SQL Server 2008 will be different, but do not expect them to be lower.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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Disclaimer: All information provided is a personal opinion that may not match reality.
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