My P965 will top at at the same FSB with 4 * 1gig (which is what I use atm) as it does with 2*2gig. My E6300 is happy to sit at 3.2Gighz, even with 4*1gig. The real question that needs to be asked is "Is my FSB going to be limited before my CPU/other component is"...in many cases, it won't.
I have to say though, having all slots populated is becoming far less of an issue as of late. I've had some brilliant results with all populated. With the latest chipsets having such a high FSB overclock ability, I've started to see even 1 slot fail to max it out. When this is the case, populating all slots has less of an effect when more voltage is added.
I'm not by any means saying that it doesn't make a difference - it does, but its no where near as bad as it used to be thankfully