I run a 6 core AMD Thuban processor on an older AM2+ motherboard with 4 GB DDR2. I use three monitors on the system: Two on a PCIE dual head graphics card with 1GB RAM, one on the native VGA graphics built into the motherboard.
My system memory is 4GB, and I use the system as a digital audio workstation. Recently, I upgraded to Adobe Audition CS6.
I recently switched to the 64 bit Windows 7 OS (actually dual booting both 32 and 64 bit). When running in 32 bit mode, I was constantly having to reclaim memory using Iolo System Mechanic, because when I had my music apps loaded, free memory would dip down to less than 512K. I was also getting some stuttering, and Audition CS6 was not loading the cores equally (i.e. it was pegging core 5 up to >95%, which would make the DAW stutter. Huge drop in performance from Audition 3.01.
Using 64 Bit Windows with Audition, Motu's Cuemix software, MIDI Ox, and a boatload of plugins, Audition loads the cores more evenly, and more importantly, I have 2GB free. Even using deep memory defragmentation on System Mechanic would not get me near to that.
The moral of the story is don't forget to include your video card ram in your ram count. My system has 5 GB ram including the video card, and the 64 bit OS has made a huge difference in performance and stability. In 32 bit Windows, the video ram eats into your 4GB limit, and even physical address extensions won't help you.