Add memory via the vSphere Client / Virtual Machine properties.

For RHEL 5 add the memory and make the new memory online (available for the kernel).

echo "online" > /sys/device/system/memory/memoryX/state

Alternatively use the script found in the link.

For RHEL6 the add memory is automatically discovered by the kernel.

For RHEL5 & RHEL6 it is not possible to hot add CPU(s) in VmWare.

http://communities.vmware.com/docs/DOC-10492