hmm well this depend on a couple of factors, in fact.
First of all, the timings, a 5+ CAS timing 800MHz will be a bit more efficient if at all [and sometimes even less efficient] than a 3CAS timings 533Mhz ram, not to speak about 3cas 667Mhz ram, which will probably be lots more efficient than a 800MHz 5CAS ram.
BUT a 800Mhz CAS 4 [or 3 if you can actually affored it] is going to be much more efficient and fast [yes, the diffrence will be VERY noticeable] than the same cas latencies ram that will run on 533/667 mhz.
Also the 800MHz speed is the most recommended for a high end machine for gaming and overclocking, as you will look around you will see all of the people who bought new machines [like The General, idiotec and.. me?
] got 800Mhz ram.