I don't know what the problem is. But I know a few things it isn't, and one of them is your power supply. Frame rates dropping to ridiculously low levels are usually due to some bottleneck in the video card, possibly driver related. But what's weird is that you have worse performance than before. I don't know the answer.
BTW I may be wrong about this but with your memory config, you entire memory might be working in single channel mode. Dual channel only works with symmetric installations. Try removing your 2GB module and see if there's any difference.