Hrrm, seems as if the ISO may be using a PAE kernel by default. PAE is a kernel that will be able to make use of more than 4GB of RAM with i686.
Ah, I see it's apparently a decision by RH. I assume there's some logic in it, though I don't know what it is.
http://jp.centos.org/modules/newbb/viewtopic.php?topic_id=26352&forum=14
See if it's possible to add PAE to the machine settings in VirtualBox. Settings=>System=>Processor
The box is already checked but the CPU doesn't have PAE capability so it's useless.
It won't be a bad thing thatthe install may do a choice of a cpu with or not the capability of PAE as it was in the past.
--- Michel Donais