Quite true although its also worth noting that nothing is really stopping you from upclockingYou could put both in, but the problem is if they are different speeds, they'll only run as fast as the slowest memery stick. So if you have a 512MB stick of PC3200 and 256MB stick of PC2700, the 512MB stick can only run at the speed of the PC2700.
clocking the slow(do we know if it is different speed ram yet?) to a faster speedUpclocking? :S