The DRAM latency will react differently at a 533MHz stream than it will at a 711MHz stream so even though the handoff strap of data stream between cpu memory controller and RAM can only speed at a certain degree there will be a difference in the Stop<-->Read<-->Write<-->Verify stages as with lower latencies the gates will switch quicker. You can think of this somewhat like being on an escalator. If you walk up the moving escalator at 4K/hr there will be a difference in your end result (reaching the top) if you walk up at 6K/hr even though the escalator is at a steady maximum stream. Think of the steps (the time it takes to put your foot cleanly on the step) as the latency.
This is only for comparitive views as it is somewhat different since there is no three stages on the step but you probably get my drift.
You will notice the extra Gig of RAM when you are gaming (Quake4/F.E.A.R. etc) and as well when you are doing multiple read/write/verifies such as database work and video decoding/encoding.
Thus it actually depends more on what duties you perform on your computer with regards to whether you should do it or not.