I suspect you need to either recompile your kernel yourself to allow using lots of memory, or if available in binary format, install one. Not a CentOS user myself, but if it's a binary distribution like I think it is, they may provide a kernel with support for high amount of memory that you can install from a reposity, for example. That's the easy way if you use a stock kernel anyway (i.e. haven't made any changes yourself). The other way, which is almost as easy, is to obtain kernel source, copy your old configuration as the base and add the needed stuff to be able to use the rest of the memory.. (I thought "high memory support" was up to 4G?)
The reason why several gigs of RAM is not supported in "standard" kernels is that at the moment most machines have <=1G of RAM, and adding support for more would just slow down the kernel (considerably if I'm right), so it's easier to just provide another kernel for those who need it.