"line that appears across the center of the screen" is most likely aembient occlusion. turn it off
i had that problem too and i also have a HD5800.
keep AA to 4 or under for the chop - lag affect
keep drawing quality to 8 - standard

runs like a deam for me now. i only have a slight glitch now.
seems like every time they do an update something new pops up.

kinda makes me mad i bought this alienware new a month b4 CE came out.
not alien wares fault i blame SE for less then par codeing.

anyway let me know how it works out