So the graphics go like matrix after turning them from medium to high?
I wouldn't be suprised because that graphics card sucks IMO. It's the same problems with all ready-built PC's
by "general stores" because they dont know shit about computers. You got high-end processor, you got 4x more memory
you would ever need, but your graphic card is huge bottle-neck for the rest of the components.
Although I dont think that is your problem, HD4650 should still be good enough to play SA on high graphics.
Have you tried to install the latest graphic drivers?