Some tips for getting the most out of a computer, beyond installing as much RAM as possible:
:: Windows itself will use some of that RAM and every open program on your computer will use some more of it, so when computing a large synth, it is helpful to close all unnecessary programs and background services and even features like Aero Glass in Windows Vista or Windows 7.
:: On a 32-bit copy of Windows, set Windows' virtual memory's paging file to its maximum of 4GB.
:: On a 64-bit copy of Windows, set Windows' virtual memory's paging file to 6GB or 8GB.
:: If you have multiple hard drives and the one that Windows is installed on is low on space, but another drive has lots of space, change Windows' 'TEMP' environment variable to point to a directory on the hard drive with lots of free space.
Those are the main things that I can think of at the moment. I'll post more if I think of it.