Re: URGENT
I doubt that adding RAM would affect the game like this, however, take it out and see what happens, if it goes away then you probably want to check the system information and event viewer to see if there are any reported errors on the system.
Ram should just appear as extra memory and it should be seamless to the game. Is it showing up in the system information that the extra memory is installed OK, have you check the device manager etc for any hardware conflicts?
The game most assuredly is not restricted to 512Mb, if you look through the forums you'll see people have some serious h/w - I've got 2Gb on mine and that's not much compared with others.