Further Clarification.

Vista's Memory Usage
Most of the perceived bloatedness of Vista is due to prefetch, a service implemented to speed up the launch of commonly used programs. If you use a single directory for your wow installs, this loads one copy of the MPQ files into memory (other wise it loads them for each directory)

User vs System Memory
As has been mentioned, this can be detrimental to performance. If one application is running which has a heavy user mode foot print then /3Gb can improve performance but in the case of 5 boxing on a single computer, you have 5 processes wanting ~300mb each, /3Gb will just reduce the amount of system mode memory available for them all and could easily impact performance.

Memory (32bit vs 64bit)
64Bit OS require 64bit drivers and run 64bit processes. Apparently (and I am fuzzy on this one), as these use 64bit addressing, twice as much memory is used to store the same value (8 bytes treated as a single unit instead of 4 bytes). This means that the OS on 64bit will reserve more memory than the OS on 32bit.

So it seems that if you have less than 3.2Gb available in 32bit windows then you will probably benefit from the upgrade, by how much will remain to be seen

WOW Process (32bit vs 64bit)
World of Warcraft is a 32bit application, as such, when running on an 64bit OS it must run inside a wow64 (Windows on Windows) this is a 32bit emulation layer and has its own overhead. Anyone familiar with NT4 will remember the original WOW and NTVDM (Virtual Dos mode) to allow 16bit applications to run in a 32bit environment. The overhead on these is not quite as large as the WOW\VDM pairing as only one emulation layer is required. Also remember that a 32bit app has the 32bit OS limitation as far as memory addressing goes (2Mb user mode and 2mb system).

SUMMARY
So if you have 4Gb physical and 3.2Gb+ RAM available in your 32bit OS don't go 64bit as you will be unlikely to benefit from increased RAM and each WOW session will have the wow64 overhead (this is shared across all applications running within the same wow64 instance)