If you're considering AMD Opteron versus Itanium for 64-bit computing, then yes, it will make a huge difference what chip you choose. But, 64-bit systems are also available from Intel that support the same applications as 64-bit Opteron does. (Look for systems with EM64T technology from Intel).
The best thing about AMD Opteron's and EM64T-enabled Intel systems is that they can also run 32-bit OS and even 32-bit applications on 64-bit operating systems, so you can transition at your own pace. Just be sure you've got device drivers for the operating sytem you intend to run on the new hardware.
Lastly, make sure you're really need 64-bit chips. Database applications on servers with more than 4GB of memory are one obvious application that needs 64-bit. High-performance computing applications with large datasets are another.
On the other hand, Web servers, e-mail servers and many other applications may not gain from the larger memory addressing of 64-bit. But they may still run faster, due to other changes in the instruction set for 64-bit (more registers, new SSE2 instructions, etc.)
This was first published in November 2005