Yes DDR2 is one of the best types of RAM available. The Radeon X550 is not that great a card (I have it on my computer) and so that is probably your main problem, not your RAM. You might need to decrease the detail setting on your game and close down software that you're running before you start the game. 1GB of RAM is good, Everest will also tell you how much you're using, under Memory, and then physical memory utulization. PC Pitstop http://www.pcpitstop.com
will tell you any problems with your PC in general. It says that the recommended requirements for supreme commander are 1GB RAM, 3Ghz processor, and 256MB video RAM (nVidia 6600 or ATI X800 or better). So therefore the problem is your Graphics card as it doesn't meet the recommended requirements of an ATI X800.
Hope that made sense.