The first thing you will probably need for using 4GB or more of RAM is a [shift to 64-bit OS][1].

Beyond that,

- [virtual machines][2] 


  [1]: http://www.codinghorror.com/blog/archives/000811.html
  [2]: http://vmtimes.com/2009/06/can-we-run-a-64-bit-operating-system-on-a-virtual-machine-running-on-an-x86-based-machine/