However, each instance of wow is able to access the amount of memory provided to the os. Even with wow being a 32bit app, the os allows it to use the amount of memory it needs. The more memory availabe to the os, the more it can allocate to the various applications. So Wow only uses what it needs, and with 5 instances of wow you would have access to a larger pool of memory and allow non-wow applications to that memory.

In my system with 5 wow clients running I am not using more than 3 gig of memory from my pool of 8 gig, thus allowing me to have other software(video capturing, browsers, itunes, etc) and the os to have access to the remaining. The less memory swapping the better overall performance.