While 64-bit support is now considered common for both Intel and AMD processors, many Linux (as well as Windows) users are uncertain whether to use a 32-bit or 64-bit operating system with there being advantages for both paths.
Read about it at Phoronix:
Looking over the results 64-bit Ubuntu was able to provide a strong advantage in the GCC benchmarks with both LAME and the Linux 2.6.19 kernel compilation. However, a slight advantage had remained with both Unreal Tournament 2004 and the LAME encoding tests in a 32-bit environment. Outside of the i386 and x86_64 analysis, it is also worth noting that at this time there are no real speed increases between Edgy Eft and Feisty Fawn; however, this was only the first Alpha (Herd) release of many to come before the April 2007 release of Ubuntu 7.04.