When you buy a 64-bit chip, it will typically run in either 32 or 64-bit mode. If you don’t buy a special 64-bit operating system and acquire special 64-bit software (such a 64-bit Java JVM), your chip will never strut its 64-bit powers.

Compare two computers with equal RAM, the same clock speed, one with a 32-bit CPU and one with a 64-bit CPU. You may be surprised to discover the 64-bit CPU will run run perhaps 15% slower. This is because every pointer consumes twice as much RAM. The main advantage of the 64-bit machine is its ability to address a huge virtual address space. A 32-bit machine addresses 4 gigabytes of RAM, but in practice often only about 1 gigabyte of that is usable for an application program’s heap.

The main reason then to go for a 64-bit machine is when you have large amounts of real RAM, and you want massive virtual RAM space so that you can memory-map huge files (i.e. treat them as part of the virual memory) or deal with millions of Java objects.

There are no single chip X86-64 solutions existing addressing more than 8 gigabytes of real RAM now. Many dual xeon boards are limited to 8 gigabytes total, but dual Opteron board can accept up to 16 gigabytes. For Opterons, just multiply the number of chips times 8 gigabytes and you get total main memory potential.

Sun’s Java supports the 64-bit Opteron under Linux.

AMD’s Athlon, Turion, Sempro and Opteron series use a different 64-bit architecture than the Intel Itanium called AMD64 or x-64. The architecture features 64 general purpose registers (vs only 8 in 32 bit mode), and an address space of 2⁴⁸ bytes or 281,475 gigabytes. The architecture is designed to be extended to support 2⁶⁴ bytes. There is a x-64 version of Windows Vista at nominal extra charge. Processing of longs and doubles will benefit from the 64-bit data paths, Because of AMD’s success with this instruction set, and the slow Itanium sales, Intel implemented AMD64 too, renaming it Intel-64 and used it in their newer Pentium, Celeron D, Xeon, and Core 2 processors.