I have a "similar" build (my processor is a lot better, but GC is only a little better) and honestly I can run on max settings (1080p) while in full screen, but it's almost unplayable while in windowed mode, which confuses me bcuz my specs should easily handle it but whatever, maybe it's like raelgun said and its just windows being windows...

Anyway, first thing first, try it in full screen mode and repost your results. My results on Maximum settings when running the benchmark on windowed mode was like 1500, and 6400 when in full screen. Now that I have the game, I play on maximum settings with no lag in full screen, and everything on.