http://serverfault.com/questions/127788/what-is-the-maximum-memory-a-process-mysql-can-consume-on-a-32-bit-os
Most modern distributions and processors support PAE, which is Intel's instruction set allowing memory addressing beyond the 32-bit limitation. PAE allows a maximum of 64GB of RAM to be accessed by the processor. PAE has additional CPU overhead.
32-bit Linux has a per process limit of 3GB addressable even with PAE. Typically, the kernel reserves approximately 1GB.
MySQL is a single process daemon. As such, it will be unable to address outside of the 32-bit limitation even with PAE.
Some useful references:
