Kevan, there are two ways to upgrade RAM, speed and size. First, you need to figure out if you're using DDR or DDR2. This is as simple as opening the case and taking the RAM out (make sure you power down first of course). There should be a label on the memory stick telling you what type of RAM it is. If not, check the board as that may also have the slots labeled as DDR or DDR2. You'll also want to check to see how much (size) your board can hold as well as the maximum speed supported (this should hopefully be with the manual; if not, contact the manufacturer).
Once you've determined the type, your capacity, and speed supported, you can start shopping. When looking around, as I've mentioned, you can buy speed or size. The size (in your current example, is 256Mb) and the speed is typically 266-550 Mhz (DDR) and 400-1200 Mhz(DDR2). I don't think your current RAM would be 128 DDR2, as DDR2 is normally 240. DDR is usually 184 I believe. Obviously the more and faster you go, the higher your price goes. If you're looking to only run one additional virtual machine, I would recommend you double to 512Mb at a minimum, preferably to 1Gb if you can afford it.
Hope that helps.