While the Microsoft reference may say that you should keep the operating system the same, it seems hard to believe that there aren’t situations where one would decide to bypass this recommendation. If you are sitting on Windows 2003 x86 with SQL Server 2000 x86 and you have the option to go to Windows 2008 x64 with SQL Server 2005 x64 then it might be a good time to move on that. Microsoft Windows 2008 R2 is the first OS to only be offered in a 64-bit version. If you want to be set for a few years, now might be time to go ahead and bite the x64 bullet. But you should also be aware of the downsides that can be associated with this decision. If you have 100 DTS Packages that primarily use Excel connections then you may eventually regret the decision to go with x64. It would also be a good idea to make a decision about going to Windows Server 2008.
RAID Levels can also be something to consider during this time. Kendal Van Dyke has an excellent series on Disk Performance that can be useful when looking at RAID levels. While you will often read that RAID 10 gives you the best bang, you may not have any buck left. Generally, it’s recommended that you stick with RAID 5 for your data files and RAID 1 for your log files. If you are not connected to a SAN and your drives are local then now is the time to get the RAID levels right.
Once you have your OS installed, its time to get it configured so that it works optimally for SQL Server. There are several key fundamentals about SQL Server installations that you will need to understand as you move forward and there are great resources from Brent Ozar and Chad Boyd included in the resource links below. Another item that Brent touches on briefly is that of Disk Partition Alignment. Jimmy May has some excellent information on that, and the SQL CAT team recently published a new whitepaper that is helpful.
SQL Server 2005 on a 64-bit Platform
32-bit vs. the 64-bit SQL Server performance surge
Q&A: Making the jump to 64-bit SQL Server 2005
Disk Performance Hands On
SQL Server Setup Checklist – Part 1
Installing Clustered SQL Servers – Outline, Checklists, Document Sheets
Disk Partition Alignment (Sector Alignment) for SQL Server: Part 1: Slide Deck
Disk Partition Alignment Best Practices for SQL Server