If you're now looking at 64-bit Computing on the Windows Server System, there's lots of information out there, if you know where to look.
For developers, there a slew of webcasts available including ones for migrating, optimising, compatibility, and insider tips to name but a few. There's also some webcasts there which are equally applicable to IT Pro's.
For IT Pro's specifically, you're probably wanting to know which applications support the Windows Server 2003 x64 Editions. The answer is here all the way from Allscripts to WRQ. I was hoping that Zondycyber MAPGIS Platform was a native 64-bit app to give me the full A-Z, but unfortunately, it's just an A-W for now :-)