It’s important to choose that operating system which support by our hardware.
So since we have x64 hardware obviously we must use 64-bit software. (64-bit Edition of server only and full installation –not server core which exchange will not run on it).
And we can use one of the below server Edition.
- Windows server 2008 Standard Edition (service pack 2)
- Windows server 2008 Enterprise Edition (service pack 2)
- Windows server 2008 Standard R2
- Windows server 2008 Enterprise R2