64-bit processors are quite common, e.g. [Digital Alpha]?, versions of [Sun SPARC]?, and the [IBM AS/4000]?. The PowerPC and Intel are expected to move to 64 bits at their next generation - [PPC 620]? and [Intel P7]?.
A 64-bit address bus allows the processor to address 18 million gigabytes as opposed to the mere 4 gigabytes allowed with 32 bits. Floating point calculations can also be more accurate.
Often mixed architectures are used: with [32 bit]? integer/addresses and 64bit floats.
A 64-bit operating system is needed to take full advantage of a 64-bit CPU, but a 32-bit OS can run on it.