if you do not have one of the boards that downclocks 4x512mb is better because you can get lower latency sticks. generally the best overclocking chips only come as 512mb sticks as well
That a load of crap.
The memory would have to be run in 2T then.
Also stablility would decrease and so would overclockability.
Looser timings would have to be used just to gain stable performance.