"SharePoint 2007 to 2010 Upgrade" online project (part 4): "Preparing to install SharePoint 2010. Prerequisites"

The Best Practices are from a book: Microsoft SharePoint 2010 Administrator's Companion *

Best Practice: "People often perform upgrades of existing operating system and software applications. A best practice is to make sure that all components of your SharePoint farm are always fresh installs and not upgrades"

Hardware requirements:

Web\App server:
CPU: 64-bit, 4 core
RAM: 8 Gb
System drive: minimum 80 GB (Why so? - Read on SharePoint 2010 and the C Drive)

Best Practice: "In a production environment, additional free space should equal at least two times that amount of RAM on the server to provide additional space for normal day-to-day operations and for memory dumps"

SQL server:
CPU: 64-bit, 4 core(minimum)
RAM: 8 Gb
System drive: minimum 80 GB

Software requirements

The 64-bit edition Windows Server 2008 SP2

Or

The 64-bit edition Windows Server 2008 R2

The 64-bit edition of Microsoft SQL Server 2008 R2.
Or
The 64-bit edition of Microsoft SQL Server 2008 with Service Pack 1 (SP1) and Cumulative Update 2
Or
The 64-bit edition of Microsoft SQL Server 2005 with Service Pack 3 (SP3)

For WFE:
You have to have .net Framework 3.5 SP1

For client machines:

 - It's good to know that SharePoint 2010 has a concept of Level 1 and 2 Web browser supportability.
Level 1 ( support all Active X features) - IE; Level 2 - other browsers (support basic functionality)
 - For a richer user experience Silverlight version 3 or higher;

Best Practice: It's strongly recommended that you use a dedicated account to log on and install SharePoint 2010. Don't use your administrator account!

There is a SharePoint 2010 Prerequisite Installer which can automatically install the missing prerequisite software.

In a multiple server farm, it is extremely important to run the prerequisite installer and SharePoint 2010 Setup.exe on ALL servers in the farm before running the SharePoint Products Configuration Wizard on any server in the farm!

* Even though the book is published by Microsoft Press in 2011, it's lacking consistency with MSDN in some places. I have included the quotes from the book that don't contradict MSDN and my opinion. The hardware requirements are from MSDN with a little bit explanation from blogosphere)