Don't worry about Windows Index rating, seriously, its one of the worst most inconsistent benchmarking tools there is.
That was a "blue-screen" which means you probably had a hardware error. This happens frequently with unstable overclocks and faulty RAM. Although your GPU could be the cause of it.
DXDIAG says my memory is 4096mb on my GPU, when its only 768mb. I'm not sure how it calculates it, and I used to know the reason, but just ignore it it doesn't read it "correctly".
First thing I think of is, do you have the latest drivers installed from AMDs website?
EDIT - Here is the answer for the DXDIAG problem, basically it just doesn't show the exact memory.http://www.computerhope.com/issues/ch001308.htm