I dropped 4x2GB into a Mac Pro this morning and have had some very eye-opening experiences. To put it succintly: Mac OS X can and will use all available memory (unlike Windows) and the net effect is many operations are dramatically sped up. Snapshots used to take 30-40s and are very, very fast now (like 5-7s). If you do heavy multitasking with a lot of apps running you might do some research to gauge how much memory is really enough. The other possibility is that going from 8 dimms back to 4 dimms eliminates the serialization on the memory controller and puts it back into quad mode (or dual, whatever the wide addressing is).
