Short answer: yes. That is, as long is the RAM is otherwise compatible with your system, it'll be just fine. For example, my firewall is an old AMD K6-2 ~300MHz with 128mb SDRAM, and that 128 is made up of 3 different sticks.
With DDR the same is true, but it's best to match the sticks as closely as possible, especially if you have a dual-channel motherboard. Otherwise you may have performance or even stability problems.