If your using Parallels (which you should ^^) or VMWare Fusion (which is very similar to parallels) you just select your already installed Windows partition and press "Configure" or something similar under the Virtual Machine tab. Then you go to the "RAM" or "memory" tab and increase size.
For anyone using Boot Camp (Apple's solution for running Windows), check out CampTune, a free partition program that is specifically made to size your Boot Camp partition.