You can click http://support.intel.com/support/processors/ here to see all the processors that support 64-bit OS such as WinServer2008, typically you see more 64-bit in the enterprise and 32-bit on the desktop and mobile platforms. Thanks for your question.
Find attached list of Xeon processors that are 64bit supported. Minimum processor frequency is 1.6Ghz with a cache of 4MB and 2 cores.
However your question was very generic and I supposed that since you will running Windows Server 2008 as a Server, I looked up the Xeon processors for you. These Xeons are the ones that are currently available in market however looking into archived processors that are no more easily available in market, we can revise the list. I must also state that if Xeon is not your choice, you must state the choice of your processor and I will give you the list accordingly. Hope this helps, let us know!