DDR2 RAM is the older Vesion of DDR3 I can't go into specifics, as I don't know them but it's newer and cooler and stuff.
You say that about the Sim's but if you have a 'mid range' setup it struggles if you have more than one expansion. it takes up a stupid amount of resources even on silly settings I've had to buy myself a fairly high end laptop just to be able to survive as I have every expansion pack lol, my Macbook always struggled (using windows 7) after the second expansion, kept blue screening after taking forever to load ;(
but as said, need more ram and definitely a more recent graphics card if you want it to last a while. My PC has the GTX570 GLH edition and still runs pretty strong after 2 years of having it, runs anything I throw at it with ultra high settings and looks smooth.
I'll figure it out later...