If it's a 32bit machine, the options for memory include something like <4GB, 4GB, or 64GB; the first two options would/should not even report 8GB, so chances are good that you've got the 64GB model selected. If this is the case however, it *should* report all 12GB.
If the machine is 64bit, then this is irrelevant; there are no such PAE memory options.
According to Wikipedia
however, the amount of usable/available memory when using PAE for a 32bit machine is also subject to the CPU in use on the machine. Not every CPU necessarily supports 100% the PAE extended addressing.
As for RT-ness of the kernel, I cannot comment one way or the other; I have absolutely no experience on that front, so I don't know if it affects memory addressing or not (but I'd think not..).