I recently went from 2 to 4 gigs. I learned the hard way unless you have a 64 bit system (and a 64 bit prossesor) that a windows 32 bit system will not be able to use all 4 gigs depending on Video card ram, and other sytem configurations. Averages about 3 gigs on vista 32 bit.

SH4 uses about 2 gigs, on mine of an avalible 2815MB. With everything normally running, it still only uses about 77% of avalible RAM.