What graphic settings are you trying to run the game at? I know you said it was running smoothly before, but I don't think your system could run that game smoothly on high settings, and medium may even give it some trouble.
Here's a test you can do. Take out the original 1gb of ram, and just run your system with the new 2gb of ram. If you're still having problems, run memtest memtest86.com
with only the new 2gb of ram in it. You'll need to download and then burn the iso image to a disc. You must then boot off the disc. You should let the test run for a few hours, or preferably overnight. If you ened help with this process, let us know.
"I think there is a world market for maybe five computers." - Thomas Watson, Chairman of IBM, 1943