It is possible to hot-add RAM and CPU (useful in virtual environment) without rebooting the machine.
The following commands can be used.
For the RAM (as root):
for i in $(grep -l offline /sys/devices/system/memory/*/state); do echo online > $i; done
For the CPU (as root):
for i in $(grep -l 0 /sys/devices/system/cpu/*/online); do echo 1 > $i; done
If the RAM or CPU amounts are reduced, it is of course advised to do it after shutting down the machine.