If I'm upgrading from an Athlon X2 AM2+ cpu to an X4 Phenom AM2+ CPU, do i need to reinstall my operating system? (I'm running Windows Server Standard 2008 x64)
I seriously doubt it. I would be prepared to install the OS, just in case.
Even if the machine boots, you may see significant performance hits - I've seen cases where the machine should have been faster from a new CPU, but just orders of magnitude slower.
That said, code has improved in Windows 2008 - you may be able to skip it. Do a backup, prepare to reinstall. See how the OS goes after the new installation, and then decide if you want to reinstall.
You should be able to simply swap out the CPUs and it will install the drivers on boot (if not a repair from the install CD should fix it). You might need to re-active windows though, depending on if Windows thinks it was a 'significant' hardware change.
Most likely not.
I don't think you will have to re-install.
You may have to re-activate though (this depends what license of Windows Server 2008 you are using).