Read this carefully and repeat after me "64-bit is the future, 64-bit is the future".  If you are playing (testing) Longhorn (Windows Server 2008) then use the  64-bit version.  Why, well SBS & Centro (I guess) will be 64-bit only, Windows 2008 will have a 32-bit version primarily to enable people to migrate (since it is the last version it will not be the most future proofed version IMHO).  This is a great way to prepare for the future, so just do it!!

"Windows Server 2008 is the last 32-bit operating system that we'll produce," Laing then pronounced. "Post-2008, we will transition to 64-bit. Many Microsoft products are becoming 64-bit only today, because they're realizing the benefits of 64-bit computing. Exchange Server 2007, Windows Compute Cluster Server, and Windows Server Virtualization are all 64-bit only today, because they give significant benefits."

Source: BetaNews | Windows Server 2008 'The Last 32-bit Operating System'

ttfn

David