You can address up to 36 bits of address space on a 32 bit OS (including some versions of win2k, winxp, win2003 and vista) if you enable
PAE. Doing this tends to result in unexpected results (and crashes) from poorly coded programs, though, and is generally not recommended.
There is another hack that puts the kernel reserved address space into the upper 1 GB of physical address space (instead of the default upper 2 GB), thus allowing individual programs to use 3 GB of physical RAM instead of 2 GB. This has even more dire results, given that almost all programs are coded to expect the kernel functions to reside in the upper half of the 32 bit address space, and is even less recommended.
In summation, just be happy with the 3.1 to 3.6 GB of RAM you are able to use in your system. You wouldn't see a lick of difference from being able to address the remaining bits anyways.