Thank you for your post.
FileMaker Server 8 and the current FileMaker Server 10 are 32-bit applications and run on 32-bit systems. FileMaker Server was neither tested nor certified under Windows Server 2008 in 64-bit mode; only 32-bit. Some users seem to be working successfully with FileMaker Server 10 in 64-bit mode.
If you do change to FileMaker Server 10, you do NOT have to update the clients from FileMaker Pro 8.