WoW (i really wish my pc could run it good along with alot of these games)
I dont think you should even be concerned with games until you have a machine that runs WoW. WoW is probably the most optimized for a vast degree of systems than any game free or not. Save maybe Runescape.
Not sure if it will work on your comp, but its pretty darn good in terms of a F2P MMO. Its not pay to win....yet....but time will tell what PWE does with the game after it officially launches.