Here are my settings:

CPU Q8200 quad core 2.3 GHz
4 Gb RAM
550ti

windows 7 64bit

1920 x 1080 windowed
2x MSAA
gen draw qual 9
bck draw qual 3
shadow 3
amb occ OFF
texture qual high
texture filter standard


Game runs fine for me. Got anything running in the background, ie some crappy anti virus program. I use AVG.
Maybe the issue is with the AMD.