If you really have to ask this question, then you certainly don't need it
because if you needed more than 3GiBs of ram, you would know that you need a 64bit OS.
However, this can be achieved by enabling PAE. Afaik that is possible for XP as well, I'm not sure though.
Of course, operations with large numbers are faster, because the registers are increased in size, but this does not significantly increase performance for "normal software"-usage.